This study reports on a systematic review of what, how, and why teacher educators learn. For this purpose, seventy-five research articles were analyzed. Results show that research on teacher educators' professional learning appears to be a growing field of interest but fragmented in focus. Our review indicates: (a) there is no clear knowledge base essential for teacher educators' work, (b) teacher educators undertake different activities from which to learn, and (c) they generally experience the need to learn to do their work as teacher educators. Our study may contribute to understanding and supporting teacher educators' professional learning during their career.