High school teachers today work in challenging, high-accountability instructional environments, striving to meet the needs of upwards of 100 learners per day. Rapidly growing numbers of English-language learners (ELLs) in U.S. classrooms have added to these pressures. Rather than using collaborative structures to face these challenges, the structure of departmentalization too often results in content-area high school teachers working in isolation from one another. This means that English as a Second Language (ESL) specialists, as well as other teachers of ELLs, have few if any formal structures for peer interaction across disciplines. Professional development for ELLs, when available, is still too frequently delivered in decontextualized, "one-off" sessions with little follow-up. This conflicts with the literature on best practices in professional development, which has shown, instead, how it should: (1) parallel local initiatives, standards, and teachers' own professional goals; (2) focus on the content and methods teachers use in their classrooms; (3) be sustained over time; (4) occur on-site in schools and/or in teachers' own classrooms; (5) involve collective participation of peers and colleagues; and (6) provide numerous opportunities for active learning. Aligned with these conditions of high quality, meaningful professional learning, a teacher educator, alongside content and ESL teachers at one urban high school, came together in a collaborative inquiry cycle to explore ways of improving their ELL instruction. This article shares outcomes of this teacher-led collaboration, which combined two powerful tools in professional learning--teacher-collaborative inquiry and video analysis of teaching--as a means to focus teachers across content areas on ELL pedagogy. (Contains 1 table.)