Social and Emotional Learning and Its Measurement: The ability to understand and effectively interact with others is a critical determinant of academic, social, and life success (DiPerna & Elliott, 2002). This fact is increasingly recognized in educational policy and practice. For example, an influential report by the National Academy of Sciences argued that social and emotional competencies such as self-management and social problem-solving are key to success across the lifespan (National Research Council, 2012). In recognition that these social and emotional competencies are teachable and are critical for school and life (Durlak et al., 2011; Payton et al., 2008; Benning eld et al., 2015), a growing number of states have adopted standards that describe the competencies children should know and be able to demonstrate at each grade level ( Dusenbury et al., 2018). Recent national surveys suggest that educators want to use social and emotional competence assessments to guide practice and measure growth (Atwell & Bridgeland, 2019; Hamilton et al., 2019). Despite substantial and rapid growth in the adoption of SEL policies and programs, few social and emotional competence assessments are available to support practitioners. To address the need for high-quality social and emotional competence assessments, we created SELweb LE (Late Elementary), a web-based assessment specifically designed for fourth through sixth grades. SELweb LE is designed to assess children's understanding of others' thoughts and feelings, their ability to solve social problems, and their self-control. These competencies are reflected in the widely-cited Collaborative for Academic Social and Emotional Learning (CASEL) model (https://casel.org/corecompetencies/), and they are commonly the targets of instruction in evidence-based SEL programs (Weissberg et al., 2013). This paper presents psychometric evidence of the score reliability, factor structure, criterion-related validity, and measurement equivalence of a SELweb LE. SELweb LE is designed to fulfill an unmet need for a technically sound, usable, feasible, informative, and group-administered assessment designed to measure social and emotional competencies that are described in widely-cited theories and that are the target of instruction in SEL programs and state education standards. Methods: Study samples. University IRB approval was obtained for all procedures. Three independent study samples were analyzed for this paper. The first study establishes SELweb LE's factor structure, scoring hypotheses, score reliability, and evidence of convergent, discriminant, and criterion-related validity in a sample of 1,011 fourth through sixth graders. The second study replicates SELweb LE's factor structure and score reliability in a larger sample of 12,076 students. That study tests measurement equivalence across ethnicity, sex, and language. The third study evaluates the performance of an expanded item pool and revised number of items per module, again examining factor structure, score reliability, and criterion-related validity using additional validation measures in a sample of 3,268. Measures. SELweb LE was administered in all studies. SELweb LE includes several interactive modules or subtests that provide children an opportunity to demonstrate their competence by solving challenging tasks. To assess children's understanding of others' thoughts and feelings, children read stories about others and answered questions about story characters' emotions and mental states. To assess children's social problem-solving skills, children read stories about hypothetical challenging social situations and imagined they were in the situation. They answered questions about their preferred goals, solutions, and predicted consequences of their preferred solution. To assess children's self-management skills, they were asked to imagine themselves in situations in which they felt sad, disappointed, or upset and then select an emotion regulation strategy that would help them feel better. In addition, children completed a brief self-report survey about their self-management skills. In Studies 1 and 3, a subset of children completed additional measures (Study 1 n = 288; Study 3 n = 539), including SELweb EE, an alternate assessment of social and emotional competence in Study 1, and teacher rating scales in Study 1 and 3 (SSIS in Study 1; DESSA in Study 2). Results: Analyses support SELweb LE's factor structure, score reliability, measurement equivalence, partial convergent and discriminant validity, and criterion-related validity. More broadly, evidence supports the use of SELweb LE as a measure of social and emotional competencies that are theoretically relevant and are commonly the target of universal SEL programs and practices.