This study examines the development of teachers' metastrategic knowledge (MSK), a component of metacognitive knowledge, in the context of higher order thinking. Participants were 14 junior high school and high school science teachers who attended a professional development course. Data collection was carried out by triangulation of several data sources: classroom observations, individual interviews, written assignments, and recordings of discussions that took place during the course. The Findings section provides a detailed analysis of the professional development of 2 teachers, as well as an analysis of the development of the 14 teachers as a group. The data provide evidence for the types of knowledge teachers need for applying MSK in the course of instruction, the most specific of which are MSK of thinking skills (that must be explicit) and pedagogical knowledge regarding MSK. The considerable overall development in teachers' MSK following an in-service course consisted of at least 3 different patterns of development: (a) learning MSK regarding new thinking skills, (b) transforming initially implicit metalevel knowledge into explicit metalevel knowledge, and (c) introducing changes in the class culture to value new forms of discourse regarding thinking. The implications for professional development courses in this field are discussed.