Drawing on institutional theory, this study describes how cognitive, normative, and regulative mechanisms shape bilingual teachers' language policy implementation in both English-only and bilingual contexts. Aligned with prior educational language policy research, findings indicate the important role that teachers' beliefs play in the policy implementation process. However, findings also reveal several other dimensions of the policymaking process that contribute to the coupling of language policy and classroom practice, including district-level policy, administrative oversight, and teacher training policies. This study thus offers an institutional view of language policy implementation that accounts for teachers' beliefs in a more nuanced and realistic way. Results from the study suggest the need to respond to both individual- and systems-level policy mechanisms--and the interplay between them--when designing school systems that address the educational needs of English language learners, whose presence is increasing in districts and schools across the United States.