Children's concepts develop through different stages from global and undifferentiated to concrete and abstract. In the concrete stage perceptual and physical attributes of a concept are dominant. In the abstract stage the essential mental and nonperceptual attributes of a concept are grasped. In general, children's concepts of illness shift with age from definitions based largely on feeling states to broader, more precise definitions that include specific diagnoses and psychosocial indicators of illness. The basis for determining if one is ill also shifts with age from reliance on external cues to reliance on internal body cues. Children's knowledge of specific illnesses increases markedly during the school-age years. It appears that their concepts of specific illnesses are a synthesis of real knowledge and imaginative distortions. Children's concepts of illness causation also develop through different stages from global and undifferentiated to concrete and abstract. School-age children tend to attribute illness causation to their own actions but with age illness is increasingly attributed to external causes. There is in the school-age years increasing differentiation between external and internal in explanations of illness causation with inclusion of means of internalization of the causal agent and intermediary steps between cause and effect. This trend continues in adolescence with description of the illness mechanism in terms of physiologic processes, internal organ, and malfunctions.