Given the recent emphasis on social and emotional development, many professionals who manage, develop, or influence expanded learning systems are beginning to ask, "How do we better prepare staff to promote social and emotional development?" California has adopted a statewide professional development strategy for publicly-funded expanded learning programs that is designed to raise awareness of the importance of social and emotional learning (SEL) among practitioners and build tools for the field to support implementation. The strategy -- led by a partnership among a state agency, expanded learning intermediaries, and funders -- combines leadership development, field-building initiatives, and program-level supports. It also complements the current expanded learning system. In this article, we describe the statewide strategy and discuss how it addresses workforce challenges, the core levers that California used to develop the strategy, and why and how the state-level leadership prioritized social and emotional learning. We conclude the article with lessons learned about collaboration, implementation, and assessing impact.