Individuals of all ages hold coherent, theory-like beliefs in a variety of realms that may be at odds with formal, academic theory. Research on such informal theories suggests that individuals may develop key concepts in ways that are ontologically incompatible with the same concepts as they exist in formal or academic theories. This incompatibility may be a major obstacle to learning formal versions of theories. In a study that used previous work on concept development, nine individuals were interviewed about their informal theories of child development. Subjects included three first-year university students, three experienced early childhood teachers who had 7-10 years in the classroom but were not themselves parents, and three experienced parents of 2-3 children, each in grade school. Interview questions focused on three themes: What are children like? How do children change over time? and, How do children differ from each other? Results showed that all subjects held coherent theories about child development, with important differences in ontological emphases. Students described child development as an abstraction more than teachers, who described the concept in terms of here-and-now choices made by very young children, or parents, who described the concept in terms of personal biography and long-term change. (AC)