This article reports on urban elementary teachers' understandings of cultural relevancy and the practices they enacted after a professional development on culturally relevant education (CRE) and cognitive apprenticeship. Focus group interviews support that participating teachers understood some principles of CRE but did not always match the theory to practice before our professional development. After training, video data of teaching support that this divide was mediated. These findings point to a need to engage in explicit theory-to-practice research about cultural relevancy in urban science, technology, engineering, and mathematics (STEM) teaching. Implications are provided relating to teachers planning lessons purposefully to infuse cultural relevancy into their STEM classrooms.