The notion of mathematical knowledge for teaching (MKT) rose from the need to elaborate Shulman’s conceptualization of pedagogical content knowledge (PCK) – a concept that has been developed to describe the specific domain of teacher knowledge that is specialized to teachers and their practice. MKT represents the mathematical knowledge needed by teachers to perform the demands of teaching mathematics. This academic paper is a metasynthesis of several empirical studies that adapted a qualitative methodology to inquiry for developing understanding on this research area. It starts with providing some background on MKT by discussing some of the relevant literature that maps its development spurred by the need to structure an empirically grounded framework of professional knowledge that addresses some of the major criticism surrounding PCK, and also provides an explanation of the six MKT domains. Research efforts and findings are discussed and classified between two approaches: a deficit approach that looks into how a lack of MKT among teachers affect their practice and, in some instances, student achievement; or an affordance approach that builds on the analysis of the research data to determine whether and how better MKT suggest gains or improvement on other aspects of education. From the literature review, I offer insights and raise questions about what can be done to deepen the understanding of MKT and what issues need to be addressed for research on this area to strengthen its influence on changing the classroom practice and teacher professional development.