Essentialism is the idea that items have an underlying reality that explains their manifest appearance and determines their identity. I argue 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. I consider two puzzles that this phenomenon raises: How are essentialist beliefs transmitted? And: What is the scope of essentialist reasoning?