It is well known that obstructive sleep apnea syndrome (OSAS) poses a significant public health burden as a widespread under-treated disease. The first steps to dealing with this condition are to investigate its prevalence and then to proceed to identify its risk factors. In this chapter, Dr. Carlos Zamarrón, Associate Professor of Medicine at Santiago de Compostela University (Spain), addresses the major findings on this topic by drawing on his deep knowledge of the prevalence studies conducted to date. Although methodological and age-range differences make comparisons difficult, the author stratifies and discusses them, from the early studies of Franceschi and co-workers beginning in the 1980s, to the recently published findings of Plywaczewski and colleagues. Regarding the risk factors potentially related to OSAS, there is a clear linear relationship between age and this condition. However, the author wonders whether the high prevalence of OSAS in advanced age is clinically significant, as some data suggest that OSAS observed in the elderly might be a phenomenon distinct from that in middle age. Interestingly, several studies of OSAS in older populations report little or no association of OSAS with sleepiness, hypertension or decrements in cognitive function. Concerning gender, it has long been recognized that men have greater susceptibility than women toward developing OSAS, though the author remarks that, in this syndrome, gender disparity decreases with age and by the age of 50 the incidence rates for men and women are similar. With regard to obesity, it is not only affirmed that the most relevant population-based and community-based studies confirm that excess body weight is uniformly associated with increased risk for OSAS, but it is also demonstrated that dietary and other weight-loss regimens lessen the severity of this condition, possibly by reducing body weight, though the underlying mechanism, as the author stresses, remains a matter of controversy. To finish this interesting chapter, Dr. Zamarrón focuses on some epidemiological studies on racial and ethnic prevalence patterns of OSAS, and comments on the potential role of inheritance together with several external factors such as alcohol or most sedative drugs.