Over three years, Catholic Education Melbourne introduced an Initiative to alter the way social and emotional learning (SEL) was incorporated into Melbourne’s Catholic systemic schools. The Initiative was subjected to a process of formative evaluation by the authors of this chapter. The process of formative evaluation influenced the development of the Initiative over the three years and provides a case study in how it can be used to yield contemporaneous data on the implementation of a new policy in a way that allows the implementation to be continually improved. This chapter recounts how formative evaluation was carried out and its effects on the implementation of the SEL Initiative. Additionally, it highlights key elements of successful SEL implementation.