In an effort to inform preschool policy in California, an interdisciplinary team of RAND researchers conducted a multiyear study of preschool education known as the California Preschool Study. One of the recommendations from that study was the need for a more in-depth analysis of the effectiveness of the early care and education (ECE) workforce professional development system (PDS) in California. This study aims to provide a comprehensive synthesis of what is known and what is not known about key dimensions of the state’s ECE workforce PDS and the resulting outcomes. It finds that California has taken steps to implement components of a comprehensive ECE workforce PDS but further advances are needed. In addition, information gaps mean that it is not possible to identify inefficiencies in the current system. The study provides a set of recommendations for improving the system’s effectiveness by making better use of existing resources and by improving the ability of the PDS to prepare and support the ECE workforce.