In Sleep medicine, as in most disciplines, understanding of epidemiology plays a crucial role in clinical treatment of sleep disorders. This issue discusses several recent, large epidemiologic studies with a specific focus on the clinical implications of the findings. Studies discussed include the Penn State Child Cohort, the Tucson Children's Assessment of Sleep Apnea Study, the Wisconsin Sleep Cohort Study, the Sleep Heart Health Study, the Bay Area Sleep Cohort, an ongoing study in Iceland, and the CARDIA Sleep Study.