The knowledge that mathematics teacher educators need has attracted limited but increasing attention in recent years. The papers in this special issue build on emerging themes from that work and raise additional questions that contribute towards a future research agenda in the field. Several of the articles develop conceptualisations of teacher knowledge to apply to mathematics teacher educators, introducing new aspects and drawing attention to unique characteristics of mathematics teacher educators in the process. To some extent the questions these papers raise reflect similar questions in mathematics education more generally but for which the implications for mathematics teacher educators warrant attention. In this paper we review the articles in this special issue and use them as a springboard to examine contemporary developments in the field and posit ways forward for research on mathematics teacher educators' knowledge.