Knowledge of how children come to understand the processes of causation, prevention, and treatment of illness is needed to help health professionals and educators in their work with children. Healthy children attending kindergarten through eighth grade were asked a series of standardized questions about illness, and their responses were scored on a scale corresponding to Piaget's theoretical framework of cognitive development. Children's responses varied widely at all ages, but there was a consistent systematic progression in their understanding of illness-related concepts with age. Kindergarten children typically understand illness causation as quite magical, and/or as the consequence of their transgression of rules. At fourth grade, children believe all illness to be caused by germs whose very presence is sufficient to make a child sick. The complexity of the mechanisms that must interrelate to cause illness is not understood until eighth grade at the earliest. At approximately 12 or 13 years of age children begin to understand that there are multiple causes of illness, that the body may respond variably to any or a combination of agents, and that host factors interact with the agent to cause and cure illness. This sequence in the development of understanding about illness parallels conceptual development in other content areas such as physical causality, although it seems to lag a bit. The delineation of the concepts typical of children at each developmental stage, as provided in this paper, will help to guide educational efforts and future research.