diff --git a/app/services/overlap.py b/app/services/overlap.py
new file mode 100644
index 0000000..1a5e262
--- /dev/null
+++ b/app/services/overlap.py
@@ -0,0 +1,232 @@
+"""Calendar overlap detection service.
+
+BE-CAL-006: Validates that a new or edited TimeSlot does not overlap with
+existing slots on the same day for the same user.
+
+Overlap is defined as two time ranges ``[start, start + duration)`` having
+a non-empty intersection.  Cancelled/aborted slots are excluded from
+conflict checks (they no longer occupy calendar time).
+
+For the **create** scenario, all existing non-cancelled slots on the target
+date are checked.
+
+For the **edit** scenario, the slot being edited is excluded from the
+candidate set so it doesn't conflict with its own previous position.
+"""
+
+from __future__ import annotations
+
+from datetime import date, time, timedelta, datetime
+from typing import Optional
+
+from sqlalchemy.orm import Session
+
+from app.models.calendar import SlotStatus, TimeSlot
+from app.services.plan_slot import get_virtual_slots_for_date
+
+
+# Statuses that no longer occupy calendar time — excluded from overlap checks.
+_INACTIVE_STATUSES = {SlotStatus.SKIPPED, SlotStatus.ABORTED}
+
+
+# ---------------------------------------------------------------------------
+# Internal helpers
+# ---------------------------------------------------------------------------
+
+def _time_to_minutes(t: time) -> int:
+    """Convert a ``time`` to minutes since midnight."""
+    return t.hour * 60 + t.minute
+
+
+def _ranges_overlap(
+    start_a: int,
+    end_a: int,
+    start_b: int,
+    end_b: int,
+) -> bool:
+    """Return *True* if two half-open intervals ``[a, a+dur)`` overlap."""
+    return start_a < end_b and start_b < end_a
+
+
+# ---------------------------------------------------------------------------
+# Conflict data class
+# ---------------------------------------------------------------------------
+
+class SlotConflict:
+    """Describes a single overlap conflict."""
+
+    __slots__ = ("conflicting_slot_id", "conflicting_virtual_id",
+                 "scheduled_at", "estimated_duration", "slot_type", "message")
+
+    def __init__(
+        self,
+        *,
+        conflicting_slot_id: Optional[int] = None,
+        conflicting_virtual_id: Optional[str] = None,
+        scheduled_at: time,
+        estimated_duration: int,
+        slot_type: str,
+        message: str,
+    ):
+        self.conflicting_slot_id = conflicting_slot_id
+        self.conflicting_virtual_id = conflicting_virtual_id
+        self.scheduled_at = scheduled_at
+        self.estimated_duration = estimated_duration
+        self.slot_type = slot_type
+        self.message = message
+
+    def to_dict(self) -> dict:
+        d: dict = {
+            "scheduled_at": self.scheduled_at.isoformat(),
+            "estimated_duration": self.estimated_duration,
+            "slot_type": self.slot_type,
+            "message": self.message,
+        }
+        if self.conflicting_slot_id is not None:
+            d["conflicting_slot_id"] = self.conflicting_slot_id
+        if self.conflicting_virtual_id is not None:
+            d["conflicting_virtual_id"] = self.conflicting_virtual_id
+        return d
+
+
+# ---------------------------------------------------------------------------
+# Core overlap detection
+# ---------------------------------------------------------------------------
+
+def _format_time_range(start: time, duration: int) -> str:
+    """Format a slot time range for human-readable messages."""
+    start_min = _time_to_minutes(start)
+    end_min = start_min + duration
+    end_h, end_m = divmod(end_min, 60)
+    # Clamp to 23:59 for display purposes
+    if end_h >= 24:
+        end_h, end_m = 23, 59
+    return f"{start.strftime('%H:%M')}-{end_h:02d}:{end_m:02d}"
+
+
+def check_overlap(
+    db: Session,
+    user_id: int,
+    target_date: date,
+    scheduled_at: time,
+    estimated_duration: int,
+    *,
+    exclude_slot_id: Optional[int] = None,
+) -> list[SlotConflict]:
+    """Check for time conflicts on *target_date* for *user_id*.
+
+    Parameters
+    ----------
+    db :
+        Active database session.
+    user_id :
+        The user whose calendar is being checked.
+    target_date :
+        The date to check.
+    scheduled_at :
+        Proposed start time.
+    estimated_duration :
+        Proposed duration in minutes.
+    exclude_slot_id :
+        If editing an existing slot, pass its ``id`` so it is not counted
+        as conflicting with itself.
+
+    Returns
+    -------
+    list[SlotConflict]
+        Empty list means no conflicts.  Non-empty means the proposed slot
+        overlaps with one or more existing slots.
+    """
+    new_start = _time_to_minutes(scheduled_at)
+    new_end = new_start + estimated_duration
+
+    conflicts: list[SlotConflict] = []
+
+    # ---- 1. Check real (materialized) slots --------------------------------
+    query = (
+        db.query(TimeSlot)
+        .filter(
+            TimeSlot.user_id == user_id,
+            TimeSlot.date == target_date,
+            TimeSlot.status.notin_([s.value for s in _INACTIVE_STATUSES]),
+        )
+    )
+    if exclude_slot_id is not None:
+        query = query.filter(TimeSlot.id != exclude_slot_id)
+
+    existing_slots: list[TimeSlot] = query.all()
+
+    for slot in existing_slots:
+        slot_start = _time_to_minutes(slot.scheduled_at)
+        slot_end = slot_start + slot.estimated_duration
+
+        if _ranges_overlap(new_start, new_end, slot_start, slot_end):
+            existing_range = _format_time_range(slot.scheduled_at, slot.estimated_duration)
+            proposed_range = _format_time_range(scheduled_at, estimated_duration)
+            conflicts.append(SlotConflict(
+                conflicting_slot_id=slot.id,
+                scheduled_at=slot.scheduled_at,
+                estimated_duration=slot.estimated_duration,
+                slot_type=slot.slot_type.value if hasattr(slot.slot_type, 'value') else str(slot.slot_type),
+                message=(
+                    f"Proposed slot {proposed_range} overlaps with existing "
+                    f"{slot.slot_type.value if hasattr(slot.slot_type, 'value') else slot.slot_type} "
+                    f"slot (id={slot.id}) at {existing_range}"
+                ),
+            ))
+
+    # ---- 2. Check virtual (plan-generated) slots ---------------------------
+    virtual_slots = get_virtual_slots_for_date(db, user_id, target_date)
+
+    for vs in virtual_slots:
+        vs_start = _time_to_minutes(vs["scheduled_at"])
+        vs_end = vs_start + vs["estimated_duration"]
+
+        if _ranges_overlap(new_start, new_end, vs_start, vs_end):
+            existing_range = _format_time_range(vs["scheduled_at"], vs["estimated_duration"])
+            proposed_range = _format_time_range(scheduled_at, estimated_duration)
+            slot_type_val = vs["slot_type"].value if hasattr(vs["slot_type"], 'value') else str(vs["slot_type"])
+            conflicts.append(SlotConflict(
+                conflicting_virtual_id=vs["virtual_id"],
+                scheduled_at=vs["scheduled_at"],
+                estimated_duration=vs["estimated_duration"],
+                slot_type=slot_type_val,
+                message=(
+                    f"Proposed slot {proposed_range} overlaps with virtual plan "
+                    f"slot ({vs['virtual_id']}) at {existing_range}"
+                ),
+            ))
+
+    return conflicts
+
+
+# ---------------------------------------------------------------------------
+# Convenience wrappers for create / edit scenarios
+# ---------------------------------------------------------------------------
+
+def check_overlap_for_create(
+    db: Session,
+    user_id: int,
+    target_date: date,
+    scheduled_at: time,
+    estimated_duration: int,
+) -> list[SlotConflict]:
+    """Check overlap when creating a brand-new slot (no exclusion)."""
+    return check_overlap(
+        db, user_id, target_date, scheduled_at, estimated_duration,
+    )
+
+
+def check_overlap_for_edit(
+    db: Session,
+    user_id: int,
+    slot_id: int,
+    target_date: date,
+    scheduled_at: time,
+    estimated_duration: int,
+) -> list[SlotConflict]:
+    """Check overlap when editing an existing slot (exclude itself)."""
+    return check_overlap(
+        db, user_id, target_date, scheduled_at, estimated_duration,
+        exclude_slot_id=slot_id,
+    )
diff --git a/tests/test_overlap.py b/tests/test_overlap.py
new file mode 100644
index 0000000..563edbe
--- /dev/null
+++ b/tests/test_overlap.py
@@ -0,0 +1,374 @@
+"""Tests for BE-CAL-006: Calendar overlap detection.
+
+Covers:
+  - No conflict when slots don't overlap
+  - Conflict detected for overlapping time ranges
+  - Create vs edit scenarios (edit excludes own slot)
+  - Skipped/aborted slots are not considered
+  - Virtual (plan-generated) slots are checked
+  - Edge cases: adjacent slots, exact same time, partial overlap
+"""
+
+import pytest
+from datetime import date, time
+
+from app.models.calendar import (
+    SchedulePlan,
+    SlotStatus,
+    SlotType,
+    EventType,
+    TimeSlot,
+    DayOfWeek,
+)
+from app.services.overlap import (
+    check_overlap,
+    check_overlap_for_create,
+    check_overlap_for_edit,
+)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+TARGET_DATE = date(2026, 4, 1)  # A Wednesday
+USER_ID = 1
+USER_ID_2 = 2
+
+
+def _make_slot(db, *, scheduled_at, duration=30, status=SlotStatus.NOT_STARTED, user_id=USER_ID, slot_date=TARGET_DATE, plan_id=None):
+    """Insert a real TimeSlot and return it."""
+    slot = TimeSlot(
+        user_id=user_id,
+        date=slot_date,
+        slot_type=SlotType.WORK,
+        estimated_duration=duration,
+        scheduled_at=scheduled_at,
+        status=status,
+        priority=0,
+        plan_id=plan_id,
+    )
+    db.add(slot)
+    db.flush()
+    return slot
+
+
+def _make_plan(db, *, at_time, duration=30, user_id=USER_ID, on_day=None, is_active=True):
+    """Insert a SchedulePlan and return it."""
+    plan = SchedulePlan(
+        user_id=user_id,
+        slot_type=SlotType.WORK,
+        estimated_duration=duration,
+        at_time=at_time,
+        on_day=on_day,
+        is_active=is_active,
+    )
+    db.add(plan)
+    db.flush()
+    return plan
+
+
+@pytest.fixture(autouse=True)
+def _ensure_users(seed):
+    """All overlap tests need seeded users (id=1, id=2) for FK constraints."""
+    pass
+
+
+# ---------------------------------------------------------------------------
+# No-conflict cases
+# ---------------------------------------------------------------------------
+
+class TestNoConflict:
+
+    def test_empty_calendar(self, db):
+        """No existing slots → no conflicts."""
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_adjacent_before(self, db):
+        """Existing 09:00-09:30, proposed 09:30-10:00 → no overlap."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 30), 30,
+        )
+        assert conflicts == []
+
+    def test_adjacent_after(self, db):
+        """Existing 10:00-10:30, proposed 09:30-10:00 → no overlap."""
+        _make_slot(db, scheduled_at=time(10, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 30), 30,
+        )
+        assert conflicts == []
+
+    def test_different_user(self, db):
+        """Slot for user 2 should not conflict with user 1's new slot."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, user_id=USER_ID_2)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_different_date(self, db):
+        """Same time on a different date → no conflict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, slot_date=date(2026, 4, 2))
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+
+# ---------------------------------------------------------------------------
+# Conflict detection
+# ---------------------------------------------------------------------------
+
+class TestConflictDetected:
+
+    def test_exact_same_time(self, db):
+        """Same start + same duration = overlap."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert len(conflicts) == 1
+        assert conflicts[0].conflicting_slot_id is not None
+        assert "overlaps" in conflicts[0].message
+
+    def test_partial_overlap_start(self, db):
+        """Existing 09:00-09:30, proposed 09:15-09:45 → overlap."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 15), 30,
+        )
+        assert len(conflicts) == 1
+
+    def test_partial_overlap_end(self, db):
+        """Existing 09:15-09:45, proposed 09:00-09:30 → overlap."""
+        _make_slot(db, scheduled_at=time(9, 15), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert len(conflicts) == 1
+
+    def test_proposed_contains_existing(self, db):
+        """Proposed 09:00-10:00 contains existing 09:15-09:45."""
+        _make_slot(db, scheduled_at=time(9, 15), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 50,
+        )
+        assert len(conflicts) == 1
+
+    def test_existing_contains_proposed(self, db):
+        """Existing 09:00-10:00 contains proposed 09:15-09:30."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=50)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 15), 15,
+        )
+        assert len(conflicts) == 1
+
+    def test_multiple_conflicts(self, db):
+        """Proposed overlaps with two existing slots."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        _make_slot(db, scheduled_at=time(9, 20), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 10), 30,
+        )
+        assert len(conflicts) == 2
+
+
+# ---------------------------------------------------------------------------
+# Inactive slots excluded
+# ---------------------------------------------------------------------------
+
+class TestInactiveExcluded:
+
+    def test_skipped_slot_ignored(self, db):
+        """Skipped slot at same time should not cause conflict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, status=SlotStatus.SKIPPED)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_aborted_slot_ignored(self, db):
+        """Aborted slot at same time should not cause conflict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, status=SlotStatus.ABORTED)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_ongoing_slot_conflicts(self, db):
+        """Ongoing slot should still cause conflict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, status=SlotStatus.ONGOING)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert len(conflicts) == 1
+
+    def test_deferred_slot_conflicts(self, db):
+        """Deferred slot should still cause conflict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, status=SlotStatus.DEFERRED)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert len(conflicts) == 1
+
+
+# ---------------------------------------------------------------------------
+# Edit scenario (exclude own slot)
+# ---------------------------------------------------------------------------
+
+class TestEditExcludeSelf:
+
+    def test_edit_no_self_conflict(self, db):
+        """Editing a slot to the same time should not conflict with itself."""
+        slot = _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_edit(
+            db, USER_ID, slot.id, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_edit_still_detects_others(self, db):
+        """Editing a slot detects overlap with *other* slots."""
+        slot = _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        _make_slot(db, scheduled_at=time(9, 30), duration=30)
+        db.commit()
+
+        # Move slot to overlap with the second one
+        conflicts = check_overlap_for_edit(
+            db, USER_ID, slot.id, TARGET_DATE, time(9, 20), 30,
+        )
+        assert len(conflicts) == 1
+
+    def test_edit_self_excluded_others_fine(self, db):
+        """Moving a slot to a free spot should report no conflicts."""
+        slot = _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        _make_slot(db, scheduled_at=time(10, 0), duration=30)
+        db.commit()
+
+        # Move to 11:00 — no overlap
+        conflicts = check_overlap_for_edit(
+            db, USER_ID, slot.id, TARGET_DATE, time(11, 0), 30,
+        )
+        assert conflicts == []
+
+
+# ---------------------------------------------------------------------------
+# Virtual slot (plan-generated) overlap
+# ---------------------------------------------------------------------------
+
+class TestVirtualSlotOverlap:
+
+    def test_conflict_with_virtual_slot(self, db):
+        """A plan that generates a virtual slot at 09:00 should conflict."""
+        # TARGET_DATE is 2026-04-01 (Wednesday)
+        _make_plan(db, at_time=time(9, 0), duration=30, on_day=DayOfWeek.WED)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert len(conflicts) == 1
+        assert conflicts[0].conflicting_virtual_id is not None
+        assert conflicts[0].conflicting_slot_id is None
+
+    def test_no_conflict_with_inactive_plan(self, db):
+        """Cancelled plan should not generate a virtual slot to conflict with."""
+        _make_plan(db, at_time=time(9, 0), duration=30, on_day=DayOfWeek.WED, is_active=False)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_no_conflict_with_non_matching_plan(self, db):
+        """Plan for Monday should not generate a virtual slot on Wednesday."""
+        _make_plan(db, at_time=time(9, 0), duration=30, on_day=DayOfWeek.MON)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        assert conflicts == []
+
+    def test_materialized_plan_not_double_counted(self, db):
+        """A plan that's already materialized should only be counted as a real slot, not also virtual."""
+        plan = _make_plan(db, at_time=time(9, 0), duration=30, on_day=DayOfWeek.WED)
+        _make_slot(db, scheduled_at=time(9, 0), duration=30, plan_id=plan.id)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        # Should only have 1 conflict (the real slot), not 2
+        assert len(conflicts) == 1
+        assert conflicts[0].conflicting_slot_id is not None
+
+
+# ---------------------------------------------------------------------------
+# Conflict message content
+# ---------------------------------------------------------------------------
+
+class TestConflictMessage:
+
+    def test_message_has_time_info(self, db):
+        """Conflict message should include time range information."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 15), 30,
+        )
+        assert len(conflicts) == 1
+        msg = conflicts[0].message
+        assert "09:00" in msg
+        assert "overlaps" in msg
+
+    def test_to_dict(self, db):
+        """SlotConflict.to_dict() should return a proper dict."""
+        _make_slot(db, scheduled_at=time(9, 0), duration=30)
+        db.commit()
+
+        conflicts = check_overlap_for_create(
+            db, USER_ID, TARGET_DATE, time(9, 0), 30,
+        )
+        d = conflicts[0].to_dict()
+        assert "scheduled_at" in d
+        assert "estimated_duration" in d
+        assert "slot_type" in d
+        assert "message" in d
+        assert "conflicting_slot_id" in d