This paper presents evidence of the score reliability, factor structure, criterion-related validity, and measurement equivalence of a web-based assessment of several important social and emotional competencies for children in fourth through sixth grades. The assessment, SELweb LE (Late Elementary), is designed to measure children's understanding of other's thoughts and feelings, their ability to solve social problems, and their ability to engage in self-control. SELweb LE satisfies a need for a direct assessment that measures theoretically relevant social and emotional competencies specifically for youth in the late elementary grades. The competencies measured are associated with youth outcomes, are reflected in state education standards, and are the targets of instruction in widely used social and emotional learning (SEL) curricula. Analyses support SELweb LE's factor structure, score reliability, measurement equivalence, and convergent, discriminant, and criterion-related validity. More broadly, evidence supports the use of SELweb LE to characterize student competencies. [This is the online version of an article published in "Social Development."]