The number of Spanish-speaking Dual Language Learners (DLLs) is growing rapidly in the U.S., representing an increasing share (nearly 40%) of Head Start (HS) and Early Head Start (EHS) participants as well as other early care and education (ECE) attendees. Recent developmental policy reports and HS mandates stress the importance of supporting the home language of Spanish (L1) in English-dominant child care programs as well as promoting the linguistic context in which DLL children are served. Yet, little is known whether teachers actually use L1 in ECE settings and for what purposes, and prior research has not sufficiently investigated whether the effects of programs like HS vary based on such L1 use. This mixed-methods study seeks to understand whether classroom instruction in L1 impacts Spanish-speaking DLL children's English language school readiness skills. Given current mandates that openly support classroom use of children's L1, primarily a HS sample will be used for the study, however the implications are relevant to other ECE settings as well.