Young children, especially those from lower-resource communities, often do not have adequate exposure to robust mathematics instruction in preschool. Well-organized and focused early mathematical interventions can help young children develop a foundation of informal mathematics knowledge and lead to greater mathematics knowledge upon entry into kindergarten. "Building Blocks" is a mathematics curriculum that aims to develop preschool children's knowledge of mathematics using activities that are intentionally sequenced based on the developmental progression of children's mathematical learning. The "Building Blocks" curriculum includes whole- and small-group instruction, center activities, and computer activities, as well as activities for the children's families to do at home to support classroom learning. Teachers' use of the "Building Blocks" curriculum is supported by professional development. 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 "Building Blocks" used in preschool sites on student outcomes.