Pre-K Mathematics is a supplemental mathematics program focusing on a range of mathematical concepts to help develop children’s informal mathematical knowledge. The Pre-K Mathematics program includes teacher-led, small-group mathematics activities that are engaging and hands-on, as well as caregiver-child activities that are linked to the classroom activities to support children’s math learning at home. The What Works Clearinghouse (WWC) reviews existing research on educational interventions to identify evidence-based programs and practices. This WWC intervention report summarizes the available evidence on the effects of Pre-K Mathematics on student outcomes. (author abstract)