This paper presents a two-year longitudinal study of a collaboration between four researcher-and-teacher pairs. The ultimate goal is to raise the quality of English language teaching in Hong Kong by first improving teacher understanding of language knowledge and language use and, then second, guiding teachers on how to adjust their English lesson planning accordingly. Methods used to evaluate the four teachers’ growth in understanding included comprehensive interviews at the beginning and end of the study, and stimulated recall interviews immediately following the teaching of the lesson. Findings indicate that teaching behaviours improved as a result of the collaboration. Areas of change included a deeper understanding of the concepts of language know-ledge and language use, a better balancing of language knowledge and use in lessons, a greater use of inductive teaching methodology, and a differentiated use of error correction according to the focus of the lesson – whether on language knowledge or language use. Factors contributing to change included the focus in discussions on pedagogical and content knowledge, as well as the collaborative, immediate and long-term reflective nature of intervention.[Copyright © 2011 Centre for Language Studies, National University of Singapore. The article is available online at: http://e-flt.nus.edu.sg/v8n12011/wong.htm]