The Primary Care Informatics Working Group of EFMI is working to help develop the core theory of primary care informatics (PCI). Codes, classifications, terminologies and nomenclatures form an important part of the science of PCI, as they allow clinical information to be readily stored and processed in information systems. This article provides definitions and a history of the International Classification for Primary Care (ICPC), and of the Read code and the Systematized Nomenclature for Medicine (SNOMED). The Working Group wishes to encourage shared definitions and an understanding of the practical application of structured data to improve quality in clinical practice.