The population of English learners (ELs) continues to grow in schools across the United States and around the world. In this article, we share one urban university's collaborative approach to building educational capacity for cultural and linguistic diversity through professional development efforts that brought together stakeholders from classrooms, schools, communities, and districts. This grant-funded project aimed to build educator expertise to effectively support and positively influence students' language development and disciplinary learning. Grounded in sociocultural theory, we used an apprenticeship framework of teacher development, strategically planning and implementing collaborative capacity building efforts to foster learning across individual, interpersonal, and institutional planes. In this paper, we share the results of professional development efforts across three years of this project, drawing from observation, interview, and focus group data. Findings indicate that classroom-, school-, and district-level educators developed knowledge of discipline-specific language development, pedagogical skills for effective EL teaching and learning, and leadership abilities to positively shape institutional responses to their culturally and linguistically diverse student populations. Implications focus on fostering teacher professionalism through bottom-up development of EL-specific expertise and expanded opportunities for leadership.