Mathematics teacher leadership is promising, yet complex work, and research interest in this form of leadership is on the rise. This systematic review takes stock of the current research landscape on mathematics teacher leadership, specifically mathematics specialists as teacher leaders (MSTLs), and approaches the questions: (1) What are the historical publication trends for MSTLs? and (2) What are the various methods, research questions, and framing used in research design for MSTLs? For this review, we use a subset of articles focused on in-service MSTLs that are part of a larger systematic review that includes constructs such as coaching, mathematics specialists (MS), and other forms of leading teachers. Findings provide evidence that research on MSTLs is certainly on the rise, with nearly 90% of the MSTL research originating in the last decade and published within mainly general education journals. Furthermore, the majority of MSTL studies were qualitative in nature, answered research questions centered on leader interactions with others and leader reactions to professional development programs, and used various theories and constructs to ground the work. Overall, this study extends what is known about the research surrounding MSTLs, illuminates the complexities of this work, and discusses future directions for the field.