Research on cognition and classroom practices among foreign language teachers has demonstrated the powerful influences of teachers’ prior experiences as FL learners, professional coursework, and classroom teaching practice yet has not articulated the connections among these influences. Watzke (2007) claims there is a limited understanding of the process of becoming a FL teacher, and, in particular, during the first years when practices and beliefs about the classroom context are initially solidified. In practical terms, FL departments traditionally rely on an approach for training graduate students including a one-semester methodology course on FL teaching taken either prior to or simultaneously with the first semester of teaching often representing the only substantial opportunity for development of pedagogical content knowledge. Pfeiffer (2002) contested this arrangement’s validity and explained it may shortchange graduate students who need to develop a pedagogical stance that draws on substantive pedagogy and language education and not just experiential knowledge. Based on the Sociocultural Theory of Mind this qualitative study explores the development of novice TAs’ cognition in relation to FL teaching and applies dynamic assessment procedures to the concrete practices of teacher education and supervision. The following questions frame the study: 1) Do novice TAs apply pedagogical knowledge from coursework on FL teaching to their own classroom practices and, if so, how? and 2) How can teacher supervision and specifically the use of dynamic assessment assist in the development of principled practice based on pedagogical content knowledge and not mere experiential knowledge?