Attempts to understand what contributes to teaching quality have been channeled in different directions, with two main research streams focusing on either teacher knowledge or teacher beliefs. Few are the studies that have attended to both the cognitive and the affective domain simultaneously, trying to unpack how both jointly contribute to teaching quality. Situated at the nexus of these two domains, this study aims to understand how teachers' mathematical knowledge for teaching and their pedagogical beliefs contribute to their performance in providing explanations and selecting and using tasks, as studied in a teaching simulation. Using a multiple-case approach and examining the development of three prospective teachers' knowledge and beliefs over a content-and-methods course sequence, the study documents how limitations in either knowledge or beliefs can mediate the effect of the other component on prospective teachers' performance. Implications for teacher preparation and in-service education are drawn and directions for future studies are offered.