This paper describes a study into the development of teachers' expertise, conceptualised as enhanced understanding of practice. This enhancement is detected through various indicators of the teachers' awareness of the complexity of the learning-teaching process, including purposes, beliefs, subject content, and connections between theory and practice. The study is guided by the search for evidence that discussion of what constitutes good teaching leads to a more complex understanding of practice. It starts with a description of the collaborative analysis of a mathematics lesson by a group of teachers, guided by exploring notions of good practice. From this analysis, the researchers extract evidence of improvements in understanding practice on the part of the teachers, in terms of the above indicators. We conclude that the process of characterising good practice in relation to actual samples of teaching can promote the development of teaching expertise. We also note that, in order to analyse practice, teachers require accessible theoretical tools, which allow reflection to go beyond the specifics of a particular lesson.