There has been little quantitative research on the impact of specialized teacher preparation for working with multilingual students. Yet districts, states, and the U.S. Department of Education are investing to increase the number of teachers with this specialized preparation. This article explores challenges in conducting research on this topic through discussion of an impact evaluation for Teachers Educating All Multilingual Students (TEAMS), a teacher professional development program funded by a National Professional Development grant. The researchers first explore the results of our quasi-experimental study, showing that teachers' participation in TEAMS positively impacted English learner students' English language arts assessment outcomes. They then explore tensions in meeting federal impact evaluation standards, including challenges with participant selection, capturing the impact of family and community engagement, understanding shifts in advocacy and teacher leadership, and documenting impact for dual language teachers. The authors discuss implications for research, policy, and practice.