Over the last two decades, a rising number of families with preschool age children speak a language other than English at home. Known as Dual Language Learners, or DLLs, they are likely to enter kindergarten already academically behind. This gap persists throughout high school, resulting in vastly different academic and life outcomes between DLLs and their peers. The early years present a pivotal time to interrupt this achievement gap. This gap widens as children grow, yielding vastly different life and learning outcomes for DLL students. In response to this challenge, San Mateo County's Early Childhood Language Development Institute (ECLDI) aims to ensure that early learning professionals are skilled in and committed to: (1) preserving and encouraging children's home languages and cultures; (2) developing teaching and learning supports for dual language learning; (3) creating early learning experiences, practices, and policies that are culturally and linguistically relevant; and (4) cultivating meaningful connections with families of DLLs. This effort is made in order to promote quality early learning experiences that lay the foundation for DLLs' academic and life success. This brief describes the resources, strategies, outcomes, necessary conditions, and long-term effects of the theory (theory of change) that informs ECLDI's work with DLLs. [The Theory of Change was co-developed by Soodi Ansari of ECLDI with Kendra Fehrer and Nikki Tognozzi of the Gardner Center. This three-part series shares findings from a research collaboration between the John W. Gardner Center for Youth and Their Communities at Stanford University and Early Childhood Language Development Institute (ECLDI), a program of the San Mateo County Office of Education. For the other two briefs in this series, see ED594175 and ED594176.]