Usage-based theories of language development underscore the importance of children using language to learn language. Few investigations have applied usage-based theories to dual language learners (DLLs), who comprise a growing share of children attending early education programs in the U.S. Three studies were carried out to investigate the role of DLLs' language usage during preschool. Specifically, the studies examined: 1) classrooms factors that predicted Spanish and English usage, 2) the relationships among Spanish and English usage in classrooms and children's oral language skills, and 3) the relationships among exposure to and usage of Spanish and English at home and in classrooms and children's oral language skills. Participants were selected from a larger study of language and literacy development of DLLs attending Head Start and public prekindergarten programs. Hierarchical linear models were conducted to examine predictive relationships among the variables of interest in each study. Results of the first study indicated that the amount of Spanish and English that DLLs used in classrooms was strongly influenced by how often their lead teachers and assistant teachers used Spanish and English. Findings from the second study revealed that English usage in classrooms promoted the development of oral language skills in English but negatively affected Spanish expressive vocabulary. The third study found that usage of Spanish and English was important across environmental settings, and that language usage exerted a greater effect on oral language skills than language exposure. Collectively, the results of these studies reinforce the applicability of usage-based theories to DLL children. Moreover, the results signify the importance of providing DLLs numerous opportunities to use both languages in order to promote bilingual language development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]