There is growing recognition amongst child health care practitioners of children's rights to be informed about their condition and treatment, and to be actively involved in decisions pertaining to their care. In order to facilitate such understanding, there is a need to explore the ways in which such concepts can best be communicated, in particular to younger children whose conceptual ability may be regarded as limited. Consideration of the literature on how children's conceptualisation of health and illness develops reveals diverse perspectives, with seminal work heavily influenced by the work of Piaget (1929), and a belief that the understanding of younger children may be minimal. However, more contemporary theorists refute such beliefs, suggesting that children may have far more potential to understand complex illness concepts than they have previously been given credit for. The work of Carey (1985) and Vygotsky (1962) offers alternative developmental theories congruent with this perspective, which might more appropriately underpin current practice. A variety of clinical situations are then explored in the light of this debate, including preparation of children for hospitalisation, their competence to consent, their views of pain and death, and approaches to child health promotion. Guidelines are offered to practitioners throughout, with the aim of enhancing children's understanding of their conditions, and their active participation in care, which in turn has the potential to optimise care delivery.