This article reviews the field of research and practice for arts Social-Emotional Learning (SEL) assessment. We propose areas and types of research that are needed and share the University of Wisconsin-Madison Community Arts Collaboratory as a model of mixed methodological approaches and results for developing and implementing assessments of arts SEL outcomes. The Collaboratory includes four different programs and curricula in creative writing/drama, dance, drumming, and visual arts. In light of the recent growth in SEL as valued school-based instructional goals, this article has implications for school administrators and educators as they seek to identify and analyze student-derived arts SEL data, and for policy makers who need researched-based support for allocating more resources to the arts as powerful sources of SEL data about teaching and learning.