This research is an investigation into English as a Second Language (ESL) and content area teachers' perceptions of collaboration after they completed a joint professional development program called the Collaborative Teaching Institute (CTI). The overall objective of the study was to identify how CTI and other joint professional development programs for ESL and content area teachers could better support sustained teacher collaboration. The study yielded information on the key actors, opportunities, tensions and conflicts in the collaboration between the two sets of teachers. The researchers also sought to identify specific types of activities that emerged when the teachers were successful in working together. (Contains 4 tables.)