This paper presents a taxonomy of possible synchronization relationships between pairs of items in multimedia documents. Several existing approaches to the synchronization of entire items are reviewed. We then discuss classes of synchronization based upon dynamic events or conditions occurring within media items and their internal structure. We present a taxonomy of seventy-two possible such relations, which are illustrated by numerous examples and which are formalized in the authors' temporal logic notation, Mexitl. The ideas are then applied to provide a description of the lip-synchronization problem.