Essentialism is the idea that certain categories, such as “dog,” “man,” or “intelligence,” have an underlying reality or true nature that gives objects their identity. This book argues that essentialism is an early cognitive bias. Young children's concepts reflect a deep commitment to essentialism, and this commitment leads children to look beyond the obvious in many converging ways: when learning words, generalizing knowledge to new category members, reasoning about the insides of things, contemplating the role of nature versus nurture, and constructing causal explanations. This book argues against the standard view of children as concrete or focused on the obvious, instead claiming that children have an early, powerful tendency to search for hidden, non-obvious features of things. It also disputes claims that children build up their knowledge of the world based on simple, associative learning strategies, arguing that children's concepts are embedded in rich folk theories. Parents don't explicitly teach children to essentialize; instead, during the preschool years, children spontaneously construct concepts and beliefs that reflect an essentialist bias. The book synthesizes over fifteen years of empirical research on essentialism into a unified framework and explores the broader lessons that the research imparts concerning, among other things, human concepts, children's thinking, and the ways in which language influences thought.