School districts across the United States are turning to mathematics teacher leaders (MTLs) to support the teaching and learning of mathematics. And yet, what does research seem to say about MTLs? In this paper, we report findings from an exploration of PME-NA proceedings between 1984 and 2019 to examine the role of MTLs. In particular, we examine the following: historical MTL submission trends and the extent to which these trends are coupled with the implementation of national MTL events; broad methodological trends; as well as the ways in which MTLS are positioned. Our findings indicate that future research requires explicitly describing MTLs' roles within systems of professional development to better understand their impact on practice and learning. [For the complete proceedings, see ED629884.]