This article presents a review of the major themes that have emerged in over a decade of research conducted by the authors and others on the practice of co-teaching for multilingual learners (MLs) in K-12 English language development (ELD) programs. It highlights six major themes from research and documentary accounts that focus on how teachers engage in a collaborative instructional service delivery for MLs. The authors review each of the major themes and relate them to best practices for the successful implementation of Integrated English as a New Language (ENL) instruction in New York State.