Much research has been carried out on the impact of family relationships on the development and course of different illness. Research on Expressed Emotion (EE) developed out of studies of the impact of family members on patients with schizophrenia, and has provided us with a robust measure of relatives' emotional attitudes towards patients, which has now been applied in the study of numerous psychiatric and medical illnesses. This review outlines the history of EE research in schizophrenia, and discusses the evidence for the association between family EE and the course of schizophrenic illness. Some of the factors which might moderate the association between EE and illness course are outlined and the issues of the meaning and development of EE are discussed in the light of recent theoretical advances. The application of the EE methodology in other psychiatric and medical conditions is then reviewed and conclusions are drawn about the extent to which EE predicts illness course in conditions other than schizophrenia. Consideration is given to the ways in which the application of the paradigm to a variety of illnesses or conditions with different features can enhance our understanding of the EE construct.