This book explores the role of the teacher in dual language bilingual education (DLBE) implementation in a time of nationwide program expansion, in large part due to new and unprecedented top-down initiatives at state and district level. The book provides case studies of DLBE teachers who: (a) implemented the DLBE model with fidelity; (b) struggled to implement the DLBE model; and (c) adapted the DLBE model to meet the needs of their local classroom context. The book demonstrates the way teachers as language policymakers navigate and interpret district-wide DLBE implementation and the tensions that surface through this process. The research, conducted over four years using a variety of methods, highlights the challenges and opportunities faced by teachers implementing DLBE, and will be of interest to both teachers and administrators of DLBE programs as well as scholars working in bilingual education.