Normative data, data that characterize what is usual in a defined population at a specific point or period of time, are of enormous importance to primary care physicians. Such data, which seek to describe rather than explain phenomena, are essential for: a) describing the natural history of clinical conditions in the community; b) developing standards of care for primary physicians; and c) establishing illness nosologies appropriate for primary care research. In all studies designed to obtain normative data, methodologic issues need careful attention. For some purposes, normative data may be quickly obtained in cross-sectional studies. When age effects are to be described or when time is an important consideration, longitudinal study designs may be needed to evaluate potential cohort effects and epoch effects. Especially important in studies which seek normative data are precise characterization of the study population, clear definition and measurement of phenomena, and appropriate interpretation and generalization of results.