This paper shows how models can be used by practitioners of nursing as a practical tool to analyse their own practice, in particular to assess needs and to plan, implement, and evaluate care. It is argued that the impact of existing work in theory development has been relatively small, at least in the UK, because the theories arc not sufficiently specific to everyday nursing activities and the models are over elaborate and not universally applicable. A simple model of nursing, based on a systems approach is presented and applied to one particular field of nursing practice, i.e. health visiting. The model, which can be used in any field of nursing, stresses the reciprocity of the nurse–patient relationship, and the significance of the environment in determining the concepts of nurse, patient and nursing.