This study was designed to provide a more comprehensive picture of children's understanding of illness and injury than previous studies by interviewing 83 children from 4 age groups (4/5 years, 7/8 years, 9/10 years and 11/12 years). They were asked questions about how they defined illness and health as well as questions regarding different features of specific illnesses. First, it was found that definitions of health and illness became more polarized with age. Further, significant effects of age were detected for understanding of the specific illnesses with explanations becoming more sophisticated and accurate with development. This result held for almost every illness and illness feature. Lastly, cross-illness analyses showed that children hold differing levels of understanding for each ailment, e.g. understanding of injuries was higher than illnesses. These results contribute to health psychology literature on children's understanding of specific illnesses.