This study focuses on how teachers learn to teach a new topic and the role played by their developing content knowledge as they teach. The paper is based on seven high school science teachers' studies on the teaching of semiconductors, at the time a new topic in the curriculum. Analysis of artefacts such as teacher concept maps, video recordings of lessons, journals and other classroom-based evidence shows how the extent and type of teachers' content knowledge informed their choice of teaching approaches and how their learning of content took place alongside the development of teaching strategies. The development of content knowledge was combined with increased understanding of how to teach the topic in almost all cases. Evidence of development of teachers' PCK was found in their increased ability to design teaching strategies, and their use of representations and suitable assessment tasks for their lessons. Some specific common teaching strategies were identified across the teachers. These strategies could add to the canon of teachers' topic-specific professional knowledge for semiconductors. The study provides increased understanding of how teachers simultaneously master content and its teaching and how mediated self-reflection is a fruitful approach for assisting teachers to learn to teach a new topic.