A "big idea" is a concept that gives meaning to discrete facts. In this article, the author proposes a small number of big ideas, as well as his own views on where he thinks the field needs to move to achieve its full promise in school-based behavioral and social-emotional assessment. The three big ideas he has selected include: (1) universal screening for behavioral and mental health; (2) assessing student strengths; and (3) linking assessment to intervention.