Recent changes in the organisation of primary health care have increased the range of professionals that patients may encounter, leading to renewed interest in the importance of continuity of care. To assess whether organisational changes have had an impact on continuity, it is necessary to define and measure the term. Researchers seeking to assess continuity face many conceptual and practical difficulties. This article argues that it is important to distinguish between three distinct but related concepts: longitudinal continuity from a minimum number of health professionals, caring relationships between patients and professionals, and well-coordinated care between professionals. An evaluation of Advanced Access as a case study is used to illustrate how researchers need to make several value judgements in operationalising longitudinal continuity. These include whether continuity should be measured from the perspective of patient, doctor, or healthcare system, the types of professionals and consultations that should be considered, the time period to be assessed, the measure to be used, and also practical considerations about data collection. It is argued that decisions about these issues should be based on an underlying hypothesis about why continuity may be important in the particular context. Distinguishing between longitudinal continuity, patient-professional relationships, and coordinated care makes it possible to examine interactions between these different concepts, and to examine relationships with outcomes such as patient satisfaction and quality of care. It will also give greater clarity to debates about whether new models of primary care reduce continuity.