California's CORE Districts--a consortium of eight school districts serving a racially and socioeconomically diverse population of over one million students--have since 2014 led the way in deploying measures of social and emotional learning (SEL) and school climate and culture. Influenced by surging interest and research support over the past decade, these districts have collected data in hopes of continuously improving how their K-12 schools address the social and emotional dimensions of student development. In recent years, many advocates have called for schools to pay greater attention to holistic aspects of schooling, arguing for whole-child education, attention to noncognitive factors, and programming to support student SEL. In this article, the authors describe how the CORE Districts plumbed the possibilities of using holistic measures to improve schools.