The population of students labeled as English learners (ELs), referred to here as emergent bilingual learners (EBLs), continues to grow in schools across the United States. In this article, we share one urban university's collaborative approach to building professional capacity for cultural and linguistic diversity, considering both vertical and horizontal partnerships. We conceptualize vertical collaboration as mutually beneficial partnerships between the university and schools with stakeholders prioritizing capacity building efforts for high-quality teaching and learning. Another integral and less-explored facet of partnerships, we describe the pertinent role of horizontal collaboration across universities, where teacher educators came together to define the knowledge base that practitioners of EBLs must possess to support student learning across the diverse communities of our city. Our collaborative efforts to develop a framework for teacher expertise shaped our professional development efforts in schools, as we worked with teachers and administrators to build capacity centered on the unique and diverse needs of EBLs.