Mathematics Preservice Teachers' (M-PSTs) conceptions of Mathematical Knowledge for Teaching (MKT) enhance their reflective skills because they utilize such conceptions to reflect on how to contextualize content knowledge during secondary mathematics teaching. While previous studies suggested M-PSTs develop MKT, including pedagogical content knowledge by analyzing teaching in video lessons, how M-PSTs enhance their conceptions of MKT through such analysis is underexplored. I used a collective case study approach to investigate how four secondary M-PSTs conceptualized MKT when they analyzed and discussed teaching represented in a video lesson using the MKT framework. The findings indicated that the M-PSTs often described teacher knowledge either as purely pedagogical or solely content knowledge in their initial analyses of the video lesson. After engaging in a discussion that introduced the MKT framework, the M-PSTs began to illustrate how a teacher's content knowledge influenced her instructional decisions and actions. Through this discussion, M-PSTs began identifying MKT as professional content knowledge unique to teaching and, thus, distinguished MKT from generic pedagogical or content knowledge. I argue that the language and concepts offered through the framework enhanced the M-PSTs' conceptions of MKT and suggest that the instructional activities from this study contribute to developing M-PSTs as reflective practitioners.