Preparing mainstream classroom teachers to work with culturally and linguistically diverse learners is a growing concern in education as more and more schools host increasing numbers of students whose primary language is not English. Unfortunately, significant numbers of teachers have had little preparation for working with these diverse learners and feel ill equipped to support their academic development. This mixed-methods case study explores the longitudinal impact of a professional-development program designed to increase teachers’ knowledge of second language acquisition and of appropriate instructional practices for supporting English language learners (ELLs). Findings suggest that participation in the program had a positive effect on participants’ knowledge of language and literacy acquisition, their ability to plan and manage instruction for ELLs, their understanding of appropriate assessment for ELLs, and their classroom practice. A year later, though focal participants claimed maintenance, these effects were only marginally present in their classroom practice.