"Content and Language Integrated Learning" (CLIL) aims at enhancing both the academic language awareness and academic content awareness in teachers. Such a special curriculum approach necessitates that teachers' pedagogical content knowledge (PCK) consists of distinctive categories and domains that are different from either the PCK of teachers of content subjects (e.g., mathematics, sciences or liberal studies) or that of language teachers (e.g., SLA/L2 education). Drawing on the frameworks of studies on PCK (Grossman, 1988, 1990; Shulman, 1986, 1987; Tsui, 2003) and Teacher Language Awareness (Andrews, 2003; 2006; 2007), this research starts with the assumption that effective representation of subject content in L2 English demands PCK that goes beyond simple addition of the subject matter knowledge and the pedagogical knowledge of both subject content and the English language. Such PCK inevitably involves the integration of other knowledge and skill domains that are unique to the CLIL context. More importantly, how does teacher identity change interact with change in the teacher's pedagogical content knowledge? When does a content teacher start to see him/herself also playing the partial role of an academic language teacher, and when does a language teacher start to see him/herself also playing the partial role of helping students to learn academic content? Through ethnographic case studies, this research explores the CLIL PCK as well as the CLIL teacher identity change in three content teachers (in science, mathematics, and liberal studies) as well as two English language teachers. All of them have enrolled in a part-time Master of Education programme specializing in CLIL in Hong Kong schools. The study examines and explores the transformation of the teachers' PCK as well as their identity during their participation in the teacher education programme. The implications on our understanding of the teacher knowledge base of CLIL as well as its impact on teacher identity