English as a Second Language (ESL) and mainstream teachers' collaboration has largely been presented in policy as the unproblematic sharing of ideas. However, in the 20 years since this policy has been in place within Victoria, Australia, teachers are still struggling to find ways of effectively working together. This paper presents a theoretical framework that can be used to explore the dynamics of collaboration between ESL and mainstream teachers. It will be argued that developing collaborative practices between teachers who belong to different subject disciplines and often have different views of teaching is a complex process. Collaboration requires specialised skills on the part of the ESL teacher to gain some epistemological authority within the mainstream curriculum and cross the rough ground that can divide ESL and mainstream teachers within the secondary school context.