Early care and education (ECE) programs, policymakers and practitioners across the United States are facing the reality of the increase in the population of dual language learners (DLLs). To address these children's unique and varied characteristics and experiences, ECE programs find that they must adjust and adapt their efforts. However, there is limited evidence for early childhood strategies to support DLLs and limited understanding of DLLs' development. There are increased accountability requirements for ECE programs to meet the needs of all children, including DLLs. This special section "The Development and Early Care and Education of Dual Language Learners: Examining the State of Knowledge" includes six articles reporting on the research activities of the Center for Early Care and Education Research -- Dual Language Learners (CECER-DLL). The Center was federally funded to advance the research field to improve assessment, child care, and education for DLLs from birth through five years of age. This introduction and commentary discusses the most relevant findings from the set of critical reviews of literature and the secondary analysis of the Early Childhood Longitudinal Survey-Birth Cohort conducted by the Center. Also, the major methodological challenges and research gaps found in the literature reviewed, across topic areas, are presented and recommendations for future directions to advance research on the development and early care and education of DLLs are provided. (author abstract)