Over the past two decades, the landscape of research on mathematics teacher educators (MTE) has grown considerably. One particular area of interest has focused on the knowledge needed by MTEs for their work with preservice K-8 teachers (PTs). In an effort to understand this varied landscape, we conducted an extensive review of research on frameworks of MTE knowledge. This report explores the theoretical underpinnings of MTE knowledge and highlights similarities and differences among theoretical frameworks. By mapping the terrain of research on MTEs' knowledge, our goal is to identify aspects of MTE knowledge to inform the types of research that may be needed for its further development. [For the complete proceedings, see ED630210.]