This Monograph examines the development of infants' and toddlers' knowledge about human bodies. The concept of the human body is highly complex and operates on several levels of knowing. We move our bodies and recognize similar movements in the bodies of others, we have expectations about how bodies are typically built, and we have detailed knowledge about internal and external parts of the body and their functions. In the work presented here, the authors explore how these aspects of body knowledge originate in the first few years of life, and report a series of studies that documents the development of infants' and toddlers' knowledge about the structure of the human body. Results from the studies indicate that infants' knowledge about the structure of the human body is initially highly schematic, becoming more detailed and specific in the second year of life.