The purpose of this working paper is to identify areas of empirical research knowledge and gaps in knowledge about the development of infants and toddlers who are dual language learners (DLLs). This information will inform the work of the Center for Early Care and Education Research, Dual Language Learners (CECER-DLL) on assessment and measurement as well as evidence-based practices. This paper builds on prior work of the CECER-DLL, which reviewed the literature on DLLs aged 0-5 in several domains, including cognitive, social-emotional, and language and literacy development; and early care and education (ECE) practices and measures. A common theme in those critical literature reviews was that much of the small but growing body of research on young DLLs has focused on preschool-aged children, and that more research is needed that focuses on infants and toddlers. This paper therefore draws upon the smaller body of empirical research on infants and toddlers who are DLLs, as well as research on non-DLL infants and toddlers to identify the gaps in knowledge and make recommendations for future research. (author abstract)