Epidemiological studies have contributed to our understanding of several aspects of hypertension which could not have been remarked upon by clinical medicine alone: 1) the continuous nature of the relationship between the level of hypertension and cardiovascular risk has shown that the clinical definition by criteria based on numbers is arbitrary and should be adjusted according to the context and therapeutic management; 2) the risk of high blood pressure is strongly dependent on the presence or absence of other risk factors (smoking, diabetes, serum lipids, left ventricular hypertrophy), a factor which is not sufficiently taken into account in clinical practice; 3) assessment of the absolute risk of an individual on the basis of the respective roles of the principal risk factors is a useful guide to treatment. Moreover, population studies have shown that mild increases in blood pressure, associated with a long individual risk, are responsible for a large number of cardiovascular events because of the many people affected. Preventive action on the whole population is necessary in addition to individual therapeutic intervention in the clinical setting.