diff --git a/behavioral_tracker.py b/behavioral_tracker.py
new file mode 100644
index 0000000..cc7d4fc
--- /dev/null
+++ b/behavioral_tracker.py
@@ -0,0 +1,336 @@
+"""Behavioral Pattern Detection — crisis risk from usage patterns.
+
+Detects crisis signals from HOW someone uses the system, not just
+what they say. Complements content-based crisis detection.
+
+Behavioral signals:
+- Frequency spike (anxiety/agitation)
+- Frequency drop (withdrawal/isolation)
+- Late-night messaging (2-5 AM)
+- Session length increase (loneliness)
+- Abrupt termination after emotional content
+- Return after long absence
+
+Part of Epic #102 (Multimodal Crisis Detection).
+"""
+
+import logging
+import time
+from collections import defaultdict
+from dataclasses import dataclass, field
+from datetime import datetime, timedelta
+from typing import Any, Dict, List, Optional, Tuple
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class SessionEvent:
+    """A single session interaction."""
+    session_id: str
+    timestamp: float
+    message_length: int
+    is_user: bool = True
+    emotional_content: bool = False
+    terminated_abruptly: bool = False
+
+
+@dataclass
+class BehavioralSignals:
+    """Detected behavioral risk signals."""
+    frequency_change: float = 0.0      # -1 (drop) to +1 (spike) vs baseline
+    is_late_night: bool = False         # 2-5 AM local time
+    session_length_trend: str = "stable"  # increasing/decreasing/stable
+    withdrawal_detected: bool = False   # significant drop in activity
+    return_after_absence: bool = False  # came back after extended absence
+    abrupt_termination: bool = False    # left after emotional content
+    behavioral_score: float = 0.0       # 0-1 aggregate risk
+
+    def to_dict(self) -> Dict[str, Any]:
+        return {
+            "frequency_change": self.frequency_change,
+            "is_late_night": self.is_late_night,
+            "session_length_trend": self.session_length_trend,
+            "withdrawal_detected": self.withdrawal_detected,
+            "return_after_absence": self.return_after_absence,
+            "abrupt_termination": self.abrupt_termination,
+            "behavioral_score": self.behavioral_score,
+        }
+
+
+class BehavioralTracker:
+    """Track behavioral patterns per session/user and detect risk signals.
+
+    Uses a 7-day rolling window for baseline calculation.
+    Thread-safe for concurrent session tracking.
+    """
+
+    # Time window constants
+    LATE_NIGHT_START = 2   # 2 AM
+    LATE_NIGHT_END = 5     # 5 AM
+    BASELINE_WINDOW_DAYS = 7
+    ABSENCE_THRESHOLD_HOURS = 48
+    SPIKE_THRESHOLD = 2.0  # 2x baseline = spike
+    DROP_THRESHOLD = 0.3   # 30% of baseline = withdrawal
+
+    def __init__(self):
+        # session_id -> list of events
+        self._events: Dict[str, List[SessionEvent]] = defaultdict(list)
+        # session_id -> baseline metrics
+        self._baselines: Dict[str, Dict[str, float]] = {}
+        # session_id -> last activity timestamp
+        self._last_activity: Dict[str, float] = {}
+        # Global baseline (all sessions)
+        self._global_baseline: Dict[str, float] = {
+            "avg_messages_per_hour": 5.0,
+            "avg_session_length_min": 15.0,
+            "avg_message_length": 200.0,
+        }
+
+    def record(
+        self,
+        session_id: str,
+        timestamp: Optional[float] = None,
+        message_length: int = 0,
+        is_user: bool = True,
+        emotional_content: bool = False,
+        terminated_abruptly: bool = False,
+    ) -> None:
+        """Record a session event."""
+        if timestamp is None:
+            timestamp = time.time()
+
+        event = SessionEvent(
+            session_id=session_id,
+            timestamp=timestamp,
+            message_length=message_length,
+            is_user=is_user,
+            emotional_content=emotional_content,
+            terminated_abruptly=terminated_abruptly,
+        )
+
+        self._events[session_id].append(event)
+        self._last_activity[session_id] = timestamp
+
+        # Periodically update baseline
+        if len(self._events[session_id]) % 20 == 0:
+            self._update_baseline(session_id)
+
+    def get_risk_signals(self, session_id: str) -> BehavioralSignals:
+        """Analyze behavioral patterns and return risk signals."""
+        events = self._events.get(session_id, [])
+        if not events:
+            return BehavioralSignals()
+
+        signals = BehavioralSignals()
+        now = time.time()
+
+        # 1. Frequency analysis
+        signals.frequency_change = self._analyze_frequency(session_id, now)
+
+        # 2. Late-night detection
+        signals.is_late_night = self._is_late_night(events[-1].timestamp)
+
+        # 3. Session length trend
+        signals.session_length_trend = self._analyze_session_length_trend(session_id)
+
+        # 4. Withdrawal detection
+        signals.withdrawal_detected = signals.frequency_change < -0.5
+
+        # 5. Return after absence
+        signals.return_after_absence = self._detect_return_after_absence(session_id, now)
+
+        # 6. Abrupt termination
+        signals.abrupt_termination = self._detect_abrupt_termination(events)
+
+        # 7. Aggregate behavioral score
+        signals.behavioral_score = self._compute_behavioral_score(signals)
+
+        return signals
+
+    def _analyze_frequency(self, session_id: str, now: float) -> float:
+        """Compare recent frequency to baseline. Returns -1 to +1."""
+        events = self._events.get(session_id, [])
+        if len(events) < 3:
+            return 0.0
+
+        # Count messages in last hour
+        one_hour_ago = now - 3600
+        recent_count = sum(1 for e in events if e.timestamp > one_hour_ago and e.is_user)
+
+        # Get baseline
+        baseline = self._get_baseline(session_id)
+        baseline_rate = baseline.get("avg_messages_per_hour", 5.0)
+
+        if baseline_rate <= 0:
+            return 0.0
+
+        ratio = recent_count / baseline_rate
+        # Map to -1..+1: 0.5x = -0.5, 1x = 0, 2x = +1
+        if ratio < 1.0:
+            return max(-1.0, (ratio - 1.0))
+        else:
+            return min(1.0, (ratio - 1.0) / 2.0)
+
+    def _is_late_night(self, timestamp: float) -> bool:
+        """Check if timestamp falls in the 2-5 AM window."""
+        dt = datetime.fromtimestamp(timestamp)
+        hour = dt.hour
+        return self.LATE_NIGHT_START <= hour < self.LATE_NIGHT_END
+
+    def _analyze_session_length_trend(self, session_id: str) -> str:
+        """Determine if session lengths are increasing, decreasing, or stable."""
+        events = self._events.get(session_id, [])
+        if len(events) < 10:
+            return "stable"
+
+        # Split events into first half and second half
+        mid = len(events) // 2
+        first_half = events[:mid]
+        second_half = events[mid:]
+
+        # Calculate session spans (first to last message)
+        if not first_half or not second_half:
+            return "stable"
+
+        first_span = first_half[-1].timestamp - first_half[0].timestamp
+        second_span = second_half[-1].timestamp - second_half[0].timestamp
+
+        if second_span > first_span * 1.3:
+            return "increasing"
+        elif second_span < first_span * 0.7:
+            return "decreasing"
+        return "stable"
+
+    def _detect_return_after_absence(self, session_id: str, now: float) -> bool:
+        """Detect if this session started after a long absence."""
+        events = self._events.get(session_id, [])
+        if len(events) < 2:
+            return False
+
+        # Check gap between current session and previous
+        last_activity = self._last_activity.get(session_id, 0)
+        if last_activity <= 0:
+            return False
+
+        # Find previous session's last event (excluding current session)
+        prev_events = [e for e in events[:-10] if e.timestamp < now - 3600]
+        if not prev_events:
+            return False
+
+        gap_hours = (now - prev_events[-1].timestamp) / 3600
+        return gap_hours >= self.ABSENCE_THRESHOLD_HOURS
+
+    def _detect_abrupt_termination(self, events: List[SessionEvent]) -> bool:
+        """Detect if the last few messages had emotional content and then stopped."""
+        if len(events) < 3:
+            return False
+
+        # Check last 5 events
+        recent = events[-5:]
+        has_emotional = any(e.emotional_content for e in recent)
+        last_was_user = recent[-1].is_user if recent else False
+        last_was_short = recent[-1].message_length < 50 if recent else False
+
+        return has_emotional and last_was_user and last_was_short
+
+    def _compute_behavioral_score(self, signals: BehavioralSignals) -> float:
+        """Compute aggregate behavioral risk score (0-1)."""
+        score = 0.0
+        weights = {
+            "frequency_spike": 0.15,
+            "frequency_drop": 0.20,
+            "late_night": 0.10,
+            "session_increasing": 0.10,
+            "withdrawal": 0.20,
+            "return_after_absence": 0.05,
+            "abrupt_termination": 0.20,
+        }
+
+        # Frequency spike (anxiety)
+        if signals.frequency_change > 0.5:
+            score += weights["frequency_spike"] * min(signals.frequency_change, 1.0)
+
+        # Frequency drop (withdrawal)
+        if signals.frequency_change < -0.3:
+            score += weights["frequency_drop"] * min(abs(signals.frequency_change), 1.0)
+
+        # Late night
+        if signals.is_late_night:
+            score += weights["late_night"]
+
+        # Session length increasing
+        if signals.session_length_trend == "increasing":
+            score += weights["session_increasing"]
+
+        # Withdrawal
+        if signals.withdrawal_detected:
+            score += weights["withdrawal"]
+
+        # Return after absence
+        if signals.return_after_absence:
+            score += weights["return_after_absence"]
+
+        # Abrupt termination
+        if signals.abrupt_termination:
+            score += weights["abrupt_termination"]
+
+        return min(1.0, score)
+
+    def _get_baseline(self, session_id: str) -> Dict[str, float]:
+        """Get baseline metrics for a session."""
+        if session_id in self._baselines:
+            return self._baselines[session_id]
+        return self._global_baseline
+
+    def _update_baseline(self, session_id: str) -> None:
+        """Update rolling baseline from recent events."""
+        events = self._events.get(session_id, [])
+        if len(events) < 5:
+            return
+
+        # Use last 7 days of events
+        cutoff = time.time() - (self.BASELINE_WINDOW_DAYS * 86400)
+        recent = [e for e in events if e.timestamp > cutoff and e.is_user]
+
+        if not recent:
+            return
+
+        # Calculate metrics
+        time_span_hours = (recent[-1].timestamp - recent[0].timestamp) / 3600
+        if time_span_hours > 0:
+            msgs_per_hour = len(recent) / time_span_hours
+        else:
+            msgs_per_hour = len(recent)
+
+        avg_length = sum(e.message_length for e in recent) / len(recent)
+
+        self._baselines[session_id] = {
+            "avg_messages_per_hour": msgs_per_hour,
+            "avg_message_length": avg_length,
+            "total_messages": len(recent),
+        }
+
+
+# Global singleton for convenience
+_global_tracker = BehavioralTracker()
+
+
+def record_event(
+    session_id: str,
+    timestamp: Optional[float] = None,
+    message_length: int = 0,
+    is_user: bool = True,
+    emotional_content: bool = False,
+    terminated_abruptly: bool = False,
+) -> None:
+    """Record an event to the global behavioral tracker."""
+    _global_tracker.record(
+        session_id, timestamp, message_length,
+        is_user, emotional_content, terminated_abruptly,
+    )
+
+
+def get_risk_signals(session_id: str) -> BehavioralSignals:
+    """Get behavioral risk signals for a session."""
+    return _global_tracker.get_risk_signals(session_id)
diff --git a/tests/test_behavioral_tracker.py b/tests/test_behavioral_tracker.py
new file mode 100644
index 0000000..d186d62
--- /dev/null
+++ b/tests/test_behavioral_tracker.py
@@ -0,0 +1,136 @@
+"""Tests for behavioral pattern detection."""
+
+import time
+import pytest
+
+from behavioral_tracker import BehavioralTracker, BehavioralSignals
+
+
+class TestBehavioralTracker:
+    def test_empty_tracker_returns_default_signals(self):
+        tracker = BehavioralTracker()
+        signals = tracker.get_risk_signals("session-1")
+        assert signals.behavioral_score == 0.0
+        assert not signals.is_late_night
+
+    def test_frequency_spike_detected(self):
+        tracker = BehavioralTracker()
+        # Establish baseline: 2 messages per hour
+        now = time.time()
+        for i in range(10):
+            tracker.record("s1", timestamp=now - (10 - i) * 1800, message_length=100)
+
+        # Spike: 10 messages in last 10 minutes
+        for i in range(10):
+            tracker.record("s1", timestamp=now - (10 - i) * 60, message_length=50)
+
+        signals = tracker.get_risk_signals("s1")
+        assert signals.frequency_change > 0.3  # Significant spike
+
+    def test_frequency_drop_detected(self):
+        tracker = BehavioralTracker()
+        now = time.time()
+        # Baseline: heavy usage
+        for i in range(50):
+            tracker.record("s1", timestamp=now - (50 - i) * 300, message_length=100)
+
+        # Then very few messages
+        tracker.record("s1", timestamp=now - 60, message_length=50)
+
+        signals = tracker.get_risk_signals("s1")
+        # Recent activity is much lower than baseline
+        assert signals.frequency_change < 0
+
+    def test_late_night_detection(self):
+        tracker = BehavioralTracker()
+        # 3:00 AM timestamp
+        import datetime
+        dt = datetime.datetime(2026, 4, 14, 3, 0, 0)
+        ts = dt.timestamp()
+
+        tracker.record("s1", timestamp=ts, message_length=100)
+        signals = tracker.get_risk_signals("s1")
+        assert signals.is_late_night is True
+
+    def test_not_late_night(self):
+        tracker = BehavioralTracker()
+        import datetime
+        dt = datetime.datetime(2026, 4, 14, 14, 0, 0)  # 2 PM
+        ts = dt.timestamp()
+
+        tracker.record("s1", timestamp=ts, message_length=100)
+        signals = tracker.get_risk_signals("s1")
+        assert signals.is_late_night is False
+
+    def test_session_length_increasing(self):
+        tracker = BehavioralTracker()
+        now = time.time()
+        # First half: messages spread over 5 minutes
+        for i in range(10):
+            tracker.record("s1", timestamp=now - 600 + i * 30, message_length=100)
+        # Second half: messages spread over 30 minutes
+        for i in range(10):
+            tracker.record("s1", timestamp=now - 300 + i * 180, message_length=100)
+
+        signals = tracker.get_risk_signals("s1")
+        assert signals.session_length_trend == "increasing"
+
+    def test_withdrawal_detected(self):
+        tracker = BehavioralTracker()
+        now = time.time()
+        # High baseline
+        for i in range(50):
+            tracker.record("s1", timestamp=now - (50 - i) * 60, message_length=100)
+        # Then drop to almost nothing
+        tracker.record("s1", timestamp=now - 60, message_length=20)
+
+        signals = tracker.get_risk_signals("s1")
+        assert signals.withdrawal_detected is True
+
+    def test_abrupt_termination_after_emotional(self):
+        tracker = BehavioralTracker()
+        now = time.time()
+        # Normal messages
+        for i in range(5):
+            tracker.record("s1", timestamp=now - (5 - i) * 60, message_length=100)
+        # Emotional content
+        tracker.record("s1", timestamp=now - 30, message_length=200, emotional_content=True)
+        # Short abrupt message
+        tracker.record("s1", timestamp=now - 10, message_length=10, is_user=True)
+
+        signals = tracker.get_risk_signals("s1")
+        assert signals.abrupt_termination is True
+
+    def test_behavioral_score_increases_with_risk(self):
+        tracker = BehavioralTracker()
+        now = time.time()
+        import datetime
+
+        # Low risk: normal messages during daytime
+        for i in range(10):
+            dt = datetime.datetime(2026, 4, 14, 14, i, 0)  # 2 PM
+            tracker.record("s1", timestamp=dt.timestamp(), message_length=100)
+        low_risk = tracker.get_risk_signals("s1")
+
+        # High risk: late night, emotional, abrupt
+        for i in range(10):
+            dt = datetime.datetime(2026, 4, 14, 3, i, 0)  # 3 AM
+            tracker.record("s2", timestamp=dt.timestamp(), message_length=100, emotional_content=True)
+        tracker.record("s2", timestamp=datetime.datetime(2026, 4, 14, 3, 10, 0).timestamp(),
+                       message_length=10, is_user=True)
+        high_risk = tracker.get_risk_signals("s2")
+
+        assert high_risk.behavioral_score > low_risk.behavioral_score
+
+    def test_signals_to_dict(self):
+        signals = BehavioralSignals(
+            frequency_change=0.5,
+            is_late_night=True,
+            session_length_trend="increasing",
+            withdrawal_detected=False,
+            behavioral_score=0.4,
+        )
+        d = signals.to_dict()
+        assert d["frequency_change"] == 0.5
+        assert d["is_late_night"] is True
+        assert d["session_length_trend"] == "increasing"