Do young children naturally develop the foundations of science, technology, engineering, and math (STEM)? And if so, should we build on these foundations by using STEM curricula in preschools? In this article, Douglas Clements and Julie Sarama argue that the answer to both these questions is yes. First, the authors show that young children possess a sophisticated informal knowledge of math, and that they frequently ask scientific questions, such as "why" questions. Preschoolers' free play involves substantial amounts of foundational math as they explore patterns, shapes, and spatial relations; compare magnitudes; and count objects. Moreover, preschool and kindergarten children's knowledge of and interest in math and science predicts later success in STEM. And not only in STEM: the authors show that early math knowledge also predicts later reading achievement--even better than early literacy skills do. Thus mathematical thinking, Clements and Sarama say, may be cognitively foundational. That is, the thinking and reasoning inherent in math may contribute broadly to cognitive development. Is teaching STEM subjects to preschool children effective? The authors review several successful programs. They emphasize that STEM learning for young children must encompass more than facts or simple skills; rather, the classroom should be infused with interesting, appropriate opportunities to engage in math and science. And instruction should follow research-based learning trajectories that include three components: a goal, a developmental progression, and instructional activities. Clements and Sarama also discuss barriers to STEM teaching in preschool, such as the cultural belief in the United States that math achievement largely depends on native aptitude or ability, and inadequate professional development for teachers.