Partnership and the integration of language and content teaching in English-medium schools have long been active areas of research and inquiry in applied linguistics and TESOL. However, most researchers have tended to focus on methods and techniques to use in the classroom or on the analysis of the linguistic demands of the content areas. Much less attention has been paid to researching the process of co-planning and co-teaching and to supporting the evolution of the partnership between ESL and content teachers. This paper draws on questionnaire and interview data collected as part of a school-based professional development initiative in an English-medium school in Asia that focused on developing more collaborative relationships between ESL and content/classroom teachers in a large culturally and linguistically diverse elementary school. The paper begins with an analysis of some of the underlying assumptions in current conceptualisations of effective collaboration between ESL and mainstream/content-area teachers, then presents an emerging framework that draws on teacher talk and critical discourse analysis to describe and evaluate the stages of collaboration and the different levels of its effectiveness. The implications of this research for evaluating approaches to partnership and for setting realistic goals for professional development and institutional change will also be explored.