This issue brief presents the theory of change developed for the Breakthrough Series Collaborative on Social and Emotional Learning (BSC-SEL) that was designed and implemented as part of the CCL Project. A theory of change is a powerful tool that helps explain how and why a complex intervention is expected to bring about a desired change. An intervention's theory of change is often depicted visually, showing the possible causal linkages between activities, outcomes, and impacts. But a theory of change does more than show causal pathways: It also articulates how to achieve each step in the hypothesized pathways. (author abstract)