The purpose of this experimental study was to assess the efficacy of the Nuestros Ninos School Readiness (NNSR) Professional Development Program, a 2-year program that includes an integrative approach to teacher professional development (PD) and a research-based, systematic intervention component aimed to promote language, literacy, and social-emotional development, and mathematics learning in pre-kindergarten Spanish-English dual language learners (DLLs). Across cohorts and experimental conditions, 56 preschool teachers and 340 Spanish-English DLLs from early childhood programs in California, Florida, and North Carolina participated in the study. Results indicate that the NNSR program had positive effects on the overall quality of early childhood classroom practices and on practices specifically focused on DLLs. Positive results were also found for children's outcomes. DLLs in treatment classrooms showed greater gains in expressive vocabulary in English than DLLs in control classrooms, and, when assessed in Spanish, gains were higher in receptive vocabulary, alphabet knowledge, writing and early mathematics. Issues of implementation fidelity and implications for using both languages of DLL children in instruction and assessment are discussed. (author abstract)