diff --git a/ROADMAP.md b/ROADMAP.md
index f386d71..1ec0b14 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -23,7 +23,7 @@
 - [ ] Orphaned audio detection
 - [ ] OneDrive rename detection
 - [ ] Crate classification: static vs smart/dynamic
-- [ ] Library health score
+- [x] Library health score
 
 ## v0.3 — Safe Repair
 
diff --git a/docs/design/health-engine.md b/docs/design/health-engine.md
new file mode 100644
index 0000000..040d25a
--- /dev/null
+++ b/docs/design/health-engine.md
@@ -0,0 +1,31 @@
+# Library Health Engine
+
+## Problem
+
+Raw missing-reference counts do not provide a compact view of library integrity,
+but an opaque blended score would imply confidence the current data cannot support.
+
+## Architecture
+
+The health engine produces an immutable report from the core `Library`. Its score
+is only the percentage of crate references resolved by exact filename. The report
+also exposes missing references, unique missing filenames, duplicate filename
+groups, extra duplicate files, unused tracks, and missing references with matching
+candidates.
+
+Duplicate, unused, and candidate counts are informational. They do not affect the
+score until the project has a documented and validated weighting policy. An empty
+library has no score rather than a misleading 0% or 100%.
+
+## Edge Cases
+
+- The same missing filename referenced by several crates.
+- Several disk files sharing a filename.
+- Conflict-suffixed files that are unused but may be match candidates.
+- Libraries with no crate references.
+- Tracks referenced by filename from more than one crate.
+
+## Verification
+
+Tests assert every metric, the disclosed score basis, candidate integration, and
+empty-library behavior. Analysis remains entirely read-only.
diff --git a/serato_doctor/health.py b/serato_doctor/health.py
new file mode 100644
index 0000000..e708707
--- /dev/null
+++ b/serato_doctor/health.py
@@ -0,0 +1,43 @@
+from collections import Counter
+
+from serato_doctor.matching import MatchingEngine
+from serato_doctor.models.health import HealthReport
+from serato_doctor.models.library import Library
+
+
+def analyze_health(library: Library) -> HealthReport:
+    """Calculate defensible health metrics without changing the library."""
+
+    results = library.reconcile_by_filename()
+    missing = [result for result in results if not result.exists_by_filename]
+    healthy_count = len(results) - len(missing)
+    score = (
+        round(healthy_count / len(results) * 100, 1) if results else None
+    )
+
+    disk_name_counts = Counter(track.filename for track in library.tracks)
+    duplicate_counts = [count for count in disk_name_counts.values() if count > 1]
+    referenced_names = {reference.filename for reference in library.references}
+    unused_count = sum(
+        1 for track in library.tracks if track.filename not in referenced_names
+    )
+
+    matcher = MatchingEngine(library.tracks)
+    suggested_count = sum(
+        bool(matcher.candidates_for(result.reference)) for result in missing
+    )
+
+    return HealthReport(
+        score=score,
+        total_references=len(results),
+        healthy_references=healthy_count,
+        missing_references=len(missing),
+        unique_missing_filenames=len(
+            {result.reference.filename for result in missing}
+        ),
+        disk_tracks=len(library.tracks),
+        duplicate_filename_groups=len(duplicate_counts),
+        duplicate_files=sum(count - 1 for count in duplicate_counts),
+        unused_tracks=unused_count,
+        suggested_matches=suggested_count,
+    )
diff --git a/serato_doctor/models/__init__.py b/serato_doctor/models/__init__.py
index 217820c..39d2ebc 100644
--- a/serato_doctor/models/__init__.py
+++ b/serato_doctor/models/__init__.py
@@ -1,4 +1,5 @@
 from serato_doctor.models.crate import Crate
+from serato_doctor.models.health import HealthReport
 from serato_doctor.models.library import Library
 from serato_doctor.models.match import MatchEvidence, TrackMatch
 from serato_doctor.models.reference import ReferenceResult, TrackReference
@@ -7,6 +8,7 @@ from serato_doctor.models.track import DiskTrack
 __all__ = [
     "Crate",
     "DiskTrack",
+    "HealthReport",
     "Library",
     "MatchEvidence",
     "ReferenceResult",
diff --git a/serato_doctor/models/health.py b/serato_doctor/models/health.py
new file mode 100644
index 0000000..84c0b77
--- /dev/null
+++ b/serato_doctor/models/health.py
@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass(frozen=True)
+class HealthReport:
+    """Transparent aggregate findings from a read-only library analysis."""
+
+    score: Optional[float]
+    total_references: int
+    healthy_references: int
+    missing_references: int
+    unique_missing_filenames: int
+    disk_tracks: int
+    duplicate_filename_groups: int
+    duplicate_files: int
+    unused_tracks: int
+    suggested_matches: int
+
+    @property
+    def score_basis(self) -> str:
+        return "Resolved crate references / total crate references"
diff --git a/tests/test_health.py b/tests/test_health.py
new file mode 100644
index 0000000..5e4dc8c
--- /dev/null
+++ b/tests/test_health.py
@@ -0,0 +1,56 @@
+from pathlib import Path
+
+from serato_doctor.health import analyze_health
+from serato_doctor.models.library import Library
+from serato_doctor.models.reference import TrackReference
+from serato_doctor.models.track import DiskTrack
+
+
+def reference(filename):
+    return TrackReference(
+        Path("Test.crate"), Path("/old/House") / filename, filename
+    )
+
+
+def track(filename, folder="House"):
+    path = Path("/new") / folder / filename
+    return DiskTrack(path, filename, 100, path.suffix.lower())
+
+
+def test_health_report_exposes_each_metric():
+    library = Library.build(
+        [
+            reference("Found.mp3"),
+            reference("Conflict.mp3"),
+            reference("Absent.mp3"),
+        ],
+        [
+            track("Found.mp3"),
+            track("Conflict 2.mp3"),
+            track("Conflict 2.mp3", "Backup"),
+            track("Unused.mp3"),
+        ],
+    )
+
+    report = analyze_health(library)
+
+    assert report.score == 33.3
+    assert report.score_basis == (
+        "Resolved crate references / total crate references"
+    )
+    assert report.total_references == 3
+    assert report.healthy_references == 1
+    assert report.missing_references == 2
+    assert report.unique_missing_filenames == 2
+    assert report.disk_tracks == 4
+    assert report.duplicate_filename_groups == 1
+    assert report.duplicate_files == 1
+    assert report.unused_tracks == 3
+    assert report.suggested_matches == 1
+
+
+def test_empty_library_has_no_health_score():
+    report = analyze_health(Library.build([], []))
+
+    assert report.score is None
+    assert report.total_references == 0