diff --git a/Include/internal/pystate.h b/Include/internal/pystate.h
@@ -65,6 +65,79 @@ PyAPI_FUNC(_PyInitError) _PyPathConfig_Calculate(
 PyAPI_FUNC(void) _PyPathConfig_Clear(_PyPathConfig *config);
 
 
+/* interpreter state */
+
+PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_LookUpID(PY_INT64_T);
+
+
+/* cross-interpreter data */
+
+struct _xid;
+
+// _PyCrossInterpreterData is similar to Py_buffer as an effectively
+// opaque struct that holds data outside the object machinery.  This
+// is necessary to pass between interpreters in the same process.
+typedef struct _xid {
+    // data is the cross-interpreter-safe derivation of a Python object
+    // (see _PyObject_GetCrossInterpreterData).  It will be NULL if the
+    // new_object func (below) encodes the data.
+    void *data;
+    // obj is the Python object from which the data was derived.  This
+    // is non-NULL only if the data remains bound to the object in some
+    // way, such that the object must be "released" (via a decref) when
+    // the data is released.  In that case it is automatically
+    // incref'ed (to match the automatic decref when releaed).
+    PyObject *obj;
+    // interp is the ID of the owning interpreter of the original
+    // object.  It corresponds to the active interpreter when
+    // _PyObject_GetCrossInterpreterData() was called.  This should only
+    // be set by the cross-interpreter machinery.
+    //
+    // We use the ID rather than the PyInterpreterState to avoid issues
+    // with deleted interpreters.
+    int64_t interp;
+    // new_object is a function that returns a new object in the current
+    // interpreter given the data.  The resulting object (a new
+    // reference) will be equivalent to the original object.  This field
+    // is required.
+    PyObject *(*new_object)(struct _xid *);
+    // free is called when the data is released.  If it is NULL then
+    // nothing will be done to free the data.  For some types this is
+    // okay (e.g. bytes) and for those types this field should be set
+    // to NULL.  However, for most the data was allocated just for
+    // cross-interpreter use, so it must be freed when
+    // _PyCrossInterpreterData_Release is called or the memory will
+    // leak.  In that case, at the very least this field should be set
+    // to PyMem_RawFree (the default if not explicitly set to NULL).
+    // The call will happen with the original interpreter activated.
+    void (*free)(void *);
+} _PyCrossInterpreterData;
+
+typedef int (*crossinterpdatafunc)(PyObject *, _PyCrossInterpreterData *);
+PyAPI_FUNC(int) _PyObject_CheckCrossInterpreterData(PyObject *);
+
+PyAPI_FUNC(int) _PyObject_GetCrossInterpreterData(PyObject *, _PyCrossInterpreterData *);
+PyAPI_FUNC(PyObject *) _PyCrossInterpreterData_NewObject(_PyCrossInterpreterData *);
+PyAPI_FUNC(void) _PyCrossInterpreterData_Release(_PyCrossInterpreterData *);
+
+/* cross-interpreter data registry */
+
+/* For now we use a global registry of shareable classes.  An
+   alternative would be to add a tp_* slot for a class's
+   crossinterpdatafunc. It would be simpler and more efficient. */
+
+PyAPI_FUNC(int) _PyCrossInterpreterData_Register_Class(PyTypeObject *, crossinterpdatafunc);
+PyAPI_FUNC(crossinterpdatafunc) _PyCrossInterpreterData_Lookup(PyObject *);
+
+struct _xidregitem;
+
+struct _xidregitem {
+    PyTypeObject *cls;
+    crossinterpdatafunc getdata;
+    struct _xidregitem *next;
+};
+
+
 /* Full Python runtime state */
 
 typedef struct pyruntimestate {
@@ -86,6 +159,11 @@ typedef struct pyruntimestate {
            using a Python int. */
         int64_t next_id;
     } interpreters;
+    // XXX Remove this field once we have a tp_* slot.
+    struct _xidregistry {
+        PyThread_type_lock mutex;
+        struct _xidregitem *head;
+    } xidregistry;
 
 #define NEXITFUNCS 32
     void (*exitfuncs[NEXITFUNCS])(void);