Compliance has been studied from a wide range of scientific perspectives including medicine, nursing, psychology and health economics. There is no agreement regarding a commonly accepted definition. Lack of consistency in the definition and measurement of compliance is a major problem in research which becomes more complicated in an international study. The response to the confusion over the term compliance has been to suggest and use alternative terms such as adherence, co-operation, mutuality and therapeutic alliance. These terms are ill-defined and often are used as synonyms. The purpose of this paper is to analyse definitions of the concept of compliance. Abstracts from MEDLINE have been analysed in order to identify the types of compliance research that have been carried out.