The number of Spanish-speaking Dual Language Learners (DLLs) is growing rapidly in the U.S., representing an increasing share of Head Start participants. The recent experimental Head Start Impact Study (HSIS) found that Spanish-speaking DLLs benefitted more from assignment to Head Start on some academic outcomes than monolingual-English speakers, and this dissertation aimed to understand whether classroom use of Spanish played a role in these impacts. Specifically, this mixed-methods dissertation sought to answer: 1) What child, family, and institutional factors are associated with enrollment in early care environments that use Spanish for instruction?; 2) Are there main effects of Spanish language instruction on Spanish-speaking DLL children's English academic school readiness skills?; and 3) Does Head Start differentially benefit Spanish-speaking DLL children instructed in Spanish? These research questions were answered using the two largest, nationally representative samples of Head Start children - the HSIS and the Head Start Family and Child Experiences Survey (FACES, 2009 Cohort). Results showed that whether children's first language was exclusively Spanish and whether other DLL families previously attended the ECE arrangement predicted whether DLL children enrolled in centers that used Spanish for instruction. Further, DLL children instructed in Spanish had higher English receptive vocabulary skills at the end of Head Start than those not instructed, with children who attended Head Start and instructed in Spanish having the highest scores. Using the results from these secondary data analyses, classroom observations were then conducted at four local Head Start sites to answer: 4) How is Spanish used in local Head Start classrooms, for what purposes, and how may the use of Spanish possibly contribute to DLL children's school readiness? Results showed that in accordance with Head Start's "whole child" model of development, Spanish was used to promote English oral language skills in academic, socio-emotional, and health domains as well as to strengthen the home-school partnership. Taken together, the results of this mixed-methods dissertation imply that Head Start should continue targeting their resources in ways that support the home language such as bilingual teacher and staff hiring, classroom language supports, and curriculum decisions that stress the importance of both languages. (author abstract)