diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
new file mode 100644
index 0000000..7c973ff
--- /dev/null
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -0,0 +1,271 @@
+using System.Numerics;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Physics;
+
+public enum TransitionState
+{
+    Invalid  = 0,
+    OK       = 1,
+    Collided = 2,
+    Adjusted = 3,
+    Slid     = 4,
+}
+
+public enum InsertType
+{
+    Transition       = 0,
+    Placement        = 1,
+    InitialPlacement = 2,
+}
+
+[Flags]
+public enum ObjectInfoState : uint
+{
+    None            = 0x000,
+    Contact         = 0x001,
+    OnWalkable      = 0x002,
+    IsViewer        = 0x004,
+    PathClipped     = 0x008,
+    FreeRotate      = 0x010,
+    PerfectClip     = 0x040,
+    IsImpenetrable  = 0x080,
+    IsPlayer        = 0x100,
+    EdgeSlide       = 0x200,
+    IgnoreCreatures = 0x400,
+}
+
+/// <summary>
+/// Per-object flags and properties for the transition system.
+/// ACE: ObjectInfo. Decompiled: struct at transition + various offsets.
+/// </summary>
+public sealed class ObjectInfo
+{
+    public ObjectInfoState State;
+    public float StepUpHeight   = 0.01f;  // PhysicsGlobals.DefaultStepHeight
+    public float StepDownHeight = 0.04f;
+    public bool Ethereal;
+    public bool StepDown = true;
+    public float Scale = 1.0f;
+
+    // Convenience flag checks
+    public bool Contact     => State.HasFlag(ObjectInfoState.Contact);
+    public bool OnWalkable  => State.HasFlag(ObjectInfoState.OnWalkable);
+    public bool IsViewer    => State.HasFlag(ObjectInfoState.IsViewer);
+    public bool IsPlayer    => State.HasFlag(ObjectInfoState.IsPlayer);
+    public bool EdgeSlide   => State.HasFlag(ObjectInfoState.EdgeSlide);
+    public bool PathClipped => State.HasFlag(ObjectInfoState.PathClipped);
+    public bool FreeRotate  => State.HasFlag(ObjectInfoState.FreeRotate);
+}
+
+/// <summary>
+/// Accumulated collision results for the current transition.
+/// ACE: CollisionInfo.
+/// </summary>
+public sealed class CollisionInfo
+{
+    public bool ContactPlaneValid;
+    public Plane ContactPlane;
+    public uint ContactPlaneCellId;
+    public bool ContactPlaneIsWater;
+
+    public bool LastKnownContactPlaneValid;
+    public Plane LastKnownContactPlane;
+    public uint LastKnownContactPlaneCellId;
+    public bool LastKnownContactPlaneIsWater;
+
+    public bool SlidingNormalValid;
+    public Vector3 SlidingNormal;       // XY only (Z zeroed)
+
+    public bool CollisionNormalValid;
+    public Vector3 CollisionNormal;
+
+    public bool CollidedWithEnvironment;
+    public int FramesStationaryFall;
+
+    public Vector3 AdjustOffset;
+    public List<uint> CollideObjectGuids = new();
+    public uint? LastCollidedObjectGuid;
+
+    public void SetContactPlane(Plane plane, uint cellId, bool isWater = false)
+    {
+        ContactPlaneValid = true;
+        ContactPlane = plane;
+        ContactPlaneCellId = cellId;
+        ContactPlaneIsWater = isWater;
+
+        LastKnownContactPlaneValid = true;
+        LastKnownContactPlane = plane;
+        LastKnownContactPlaneCellId = cellId;
+        LastKnownContactPlaneIsWater = isWater;
+    }
+
+    public void SetSlidingNormal(Vector3 normal)
+    {
+        SlidingNormalValid = true;
+        SlidingNormal = new Vector3(normal.X, normal.Y, 0f);
+        if (SlidingNormal.LengthSquared() > PhysicsGlobals.EpsilonSq)
+            SlidingNormal = Vector3.Normalize(SlidingNormal);
+    }
+
+    public void SetCollisionNormal(Vector3 normal)
+    {
+        CollisionNormalValid = true;
+        CollisionNormal = normal;
+    }
+}
+
+/// <summary>
+/// Movement path descriptor — tracks sphere positions in multiple
+/// coordinate frames during collision resolution.
+/// ACE: SpherePath.
+/// </summary>
+public sealed class SpherePath
+{
+    public int NumSphere = 1;
+
+    // Sphere arrays — index 0 = foot/body, index 1 = head (when NumSphere==2)
+    public readonly Sphere[] LocalSphere      = new Sphere[2] { new(), new() };
+    public readonly Sphere[] GlobalSphere     = new Sphere[2] { new(), new() };
+    public readonly Sphere[] GlobalCurrCenter = new Sphere[2] { new(), new() };
+
+    // Positions
+    public Vector3 BeginPos;
+    public Vector3 EndPos;
+    public Vector3 CurPos;
+    public Vector3 CheckPos;
+    public Quaternion BeginOrientation = Quaternion.Identity;
+    public Quaternion EndOrientation   = Quaternion.Identity;
+    public Quaternion CurOrientation   = Quaternion.Identity;
+    public Quaternion CheckOrientation = Quaternion.Identity;
+
+    // Cell tracking
+    public uint CurCellId;
+    public uint CheckCellId;
+
+    // Per-step offset
+    public Vector3 GlobalOffset;
+
+    // Step-up state
+    public bool StepUp;
+    public Vector3 StepUpNormal;
+    public bool Collide;
+
+    // Step-down state
+    public bool StepDown;
+    public float StepDownAmt;
+    public float WalkInterp = 1.0f;
+
+    // Walkable tracking
+    public bool WalkableValid;
+    public Plane WalkablePlane;
+    public float WalkableAllowance = PhysicsGlobals.FloorZ;
+
+    // Backup for restore
+    public Vector3 BackupCheckPos;
+    public uint BackupCheckCellId;
+
+    // Misc flags
+    public bool NegPolyHit;
+    public bool NegStepUp;
+    public Vector3 NegCollisionNormal;
+    public bool CheckWalkable;
+    public InsertType InsertType = InsertType.Transition;
+
+    public void SetCheckPos(Vector3 pos, uint cellId)
+    {
+        CheckPos = pos;
+        CheckCellId = cellId;
+        // Update global spheres to match new check position
+        for (int i = 0; i < NumSphere; i++)
+        {
+            GlobalSphere[i].Origin = LocalSphere[i].Origin + pos;
+            GlobalSphere[i].Radius = LocalSphere[i].Radius;
+        }
+    }
+
+    public void AddOffsetToCheckPos(Vector3 offset)
+    {
+        CheckPos += offset;
+        for (int i = 0; i < NumSphere; i++)
+            GlobalSphere[i].Origin += offset;
+    }
+
+    public void SaveCheckPos()
+    {
+        BackupCheckPos = CheckPos;
+        BackupCheckCellId = CheckCellId;
+    }
+
+    public void RestoreCheckPos()
+    {
+        SetCheckPos(BackupCheckPos, BackupCheckCellId);
+    }
+
+    /// <summary>
+    /// Initialize the path for a simple point-to-point movement.
+    /// </summary>
+    public void InitPath(Vector3 begin, Vector3 end, uint cellId,
+                         float sphereRadius, float sphereHeight = 0f)
+    {
+        BeginPos = begin;
+        EndPos = end;
+        CurPos = begin;
+        CurCellId = cellId;
+
+        LocalSphere[0].Origin = new Vector3(0, 0, sphereRadius);
+        LocalSphere[0].Radius = sphereRadius;
+
+        if (sphereHeight > 0)
+        {
+            NumSphere = 2;
+            LocalSphere[1].Origin = new Vector3(0, 0, sphereHeight - sphereRadius);
+            LocalSphere[1].Radius = sphereRadius;
+        }
+        else
+        {
+            NumSphere = 1;
+        }
+
+        SetCheckPos(begin, cellId);
+
+        // Also init CurCenter
+        for (int i = 0; i < NumSphere; i++)
+        {
+            GlobalCurrCenter[i].Origin = LocalSphere[i].Origin + begin;
+            GlobalCurrCenter[i].Radius = LocalSphere[i].Radius;
+        }
+    }
+}
+
+/// <summary>
+/// Physics constants matching the retail AC client.
+/// ACE: PhysicsGlobals. Decompiled: various DAT_ addresses.
+/// </summary>
+public static class PhysicsGlobals
+{
+    public const float EPSILON    = 0.0002f;
+    public const float EpsilonSq  = EPSILON * EPSILON;
+    public const float LandingZ   = 0.0871557f;
+    public const float FloorZ     = 0.6642f;
+    public const float DefaultStepHeight = 0.01f;
+    public const float Gravity    = -9.8f;
+    public const float MaxVelocity = 50.0f;
+    public const float DummySphereRadius = 0.1f;
+    public const int   MaxTransitionSteps = 30;  // retail uses 30, ACE uses 1000
+}
+
+/// <summary>
+/// The main collision transition orchestrator.
+/// ACE: Transition. Decompiled: CTransition.
+/// Stub class — algorithm methods added in Task 6b-6d.
+/// </summary>
+public sealed class Transition
+{
+    public ObjectInfo ObjectInfo = new();
+    public SpherePath SpherePath = new();
+    public CollisionInfo CollisionInfo = new();
+
+    // Will be populated in Task 6b:
+    // public TransitionState FindTransitionalPosition(PhysicsEngine engine, PhysicsDataCache cache) { ... }
+}