The epidemiological study of sleep has grown rapidly in the past 30 years. This growth is demonstrated by examining the publication dates of the 46 articles included i n the present review (see Table 2.1). Only 2 of these articles were published i n the 1970s, and 11 were published i n the 1980s. In contrast, 26 of the articles were published during the 1990s, and 7 were published during the year 2000 or later. Thus, a rather sparse literature has rapidly evolved into a large data set. The present chapter reviews the methodology used to obtain existing data and summarizes current findings.