The purpose of this review was to provide an overview of the descriptive epidemiology of physical activity (PA) and exercise training in the United States. Overall, there is a low prevalence of meeting the current PA guidelines in all age, sex and race/ethnic groups. Among adults the prevalence of meeting the aerobic component of the guidelines is approximately 51%, whereas the prevalence of meeting both the aerobic and muscle-strengthening guidelines is approximately 23%. Approximately 27% of high school students meet the aerobic component of the pediatric guidelines (60 minutes of daily moderate-to-vigorous activity), and the proportion of youth meeting the guidelines decreases with advancing age. Further research is required to promote physically active lifestyles across the age spectrum in order to achieve optimal health benefits for the overall population.