Hospitalized children of all ages and stages need the care of concerned knowledgeable pediatric nurses who can give them opportunities to master or have some control over what is happening to them during the stress of illness or surgery. The developmental theories of Sigmund and Anna Freud, Erikson, and Piaget explain the developmental processes that document the need for this specific nursing approach. The basic preparation of the professional nurse must include education about child growth and development and the practical implications for child care. The concept of mastery must be introduced into the nursing care of hospitalized children. Examples are given of how the nurse may promote and facilitate the child's attempts to master and exert control over the situations that occur during hospitalization. The theories of Freud, Erikson, and Piaget are used to explain the hospitalized child's behaviors at various ages and stages. Specific nursing interventions that will assist the child in his/her efforts at mastery, at a particular level of development, are suggested and identified. Questions are raised concerning the needs of parents and pediatric nurses as well as hospitalized children for mastery or control of the events that are peculiar to hospitalization. The suggestion is made that well children can be given learning experiences to help them prepare for mastery in the event of hospitalization, as well as in the everyday events encountered in the process of normal growth and development.