As co-teaching expands into the realm of student teaching, it is essential for universities and their partner districts to examine those components that are critical to the success of a co-teaching partnership. This paper will share the process used in the development of the What Makes Co-teaching Work (WMCW) instrument used to support the successful implementation of a co-teaching model of student teaching. The results of this study delineate five inter-related components critical to the success of co-teaching including planning, communication, relationship, classroom applications and co-teaching knowledge. One must not assume that these components occur naturally in the student teaching partnership. Each of these key elements must be purposefully incorporated into the co-teaching experience. As universities begin adopting a co-teaching model of student teaching, the findings presented here provide guidance for establishing strong and successful co-teaching partnerships. (Contains 2 tables and 1 figure.)