The International Classification of Primary Care (ICPC) has, since its introduction in 1987, been quite successful. Now in its second revised version, it has been translated in 22 languages, accepted by the World Health Organization (WHO) as a member of the Family of International Classifications, and is being widely used both in routine daily practice and in research. In this contribution, it is explained that ICPC was designed as a theoretical classification, and that it has especially great potential when used (1) supported by the ICPC2/ICD10 Thesaurus, (2) in sufficiently large studies to allow all classes to be observed often enough to provide reliable data, and (3) in studies based on data on episodes of care, rather than encounter data only. Under these conditions, the likelihood ratios of symptoms given a diagnosis, and of co-morbidity become available, which define the clinical content of family practice.