The incidence of nocturia episodes increases with age and is associated with interrupted sleep and distress. There are no large differences in the incidence of nocturia between men and women, but young women tend to have it more frequently than young men, and very old men have it more often than very old women. The International Continence Society has defined nocturia as 'the number of voids recorded during a night's sleep: each void is preceded and followed by sleep.' This simple and useful definition should facilitate further epidemiological study of nocturia. Although there are many direct prevalence studies of nocturia, there is a great need for more advanced analytical epidemiological data on risk factors and comorbidity. Clinicians should question patients about nocturia and should consider this symptom in a broader sense than in its traditional association with benign prostatic obstruction in men and incontinence in women. There are several classic studies of nocturia. The nature of future epidemiological research will depend on the resolution of several methodological issues. Further evaluation of nocturia is warranted because of its high incidence among the general adult population.