Collaboration between English as a Second Language (ESL) and content teachers to support multilingual students designated as English Learners is a growing but complex phenomenon in U.S. schools, and one that has continued to evolve during the pandemic and related disruptions to schooling. We draw upon ecological perspectives on language learning to consider critical questions about the relationships among collaborative practices, equity, systemic capacity, and instructional quality. Such considerations can inform educational practice and better frame empirical explorations of co-teaching and related forms of collaboration.