The goal ofthis qualitative study was to examine Québec secondary school ESL teachers' conceptualization and implementation of interdisciplinary teaching. Engestrom's (2001) theory of activity systems framed this study within a sociocultural perspective. Data collected over the second half of a school-year were mainly gathered through in-class observations and interviews with six ESL teachers who were implementing interdisciplinary teaching, their school administrators, a curriculum consultant of the school board, and selected students. The findings identified three problems with the way the interdisciplinary projects were structured: (a) sequential versus simultaneous implementation of the different subject components of the interdisciplinary projects, (b) the amount of English language usage in the class, and (c) the task design of the English component of the interdisciplinary projects. The findings identified the most important constraints to the implementation of interdisciplinary teaching included a lack of scheduled common planning times with interdisciplinary partners and a dearth of professional development opportunities, material, or documents relating to interdisciplinary teaching.