Purpose: Over the past two decades, a consensus has formed that increasing physical activity and reducing sedentary behavior in older adults are important for physical and cognitive health. Although there is strong evidence that regular physical activity can prevent or delay the onset of many chronic diseases, a major concern is ensuring that older adults take part in adequate levels of physical activity. Design and Methods: This article describes the amount of physical activity prescribed between 1980 and 2005 to sedentary older adults enrolled in randomized controlled trials (RCTs) using MEDLINE, Health and Psychological Instruments, EBM Reviews, CINAHL, ERIC, PsychInfo, and Social Science Abstracts with the key words "exercise," "physical activity," and "older adult." More than 13,502 research abstracts were reviewed, and 160 RCTs 12 weeks or more in duration with documented outcomes of physical activity were synthesized. Results: The average prescribed dose of aerobic activity provided by interventions for older adults was less than the recommended amount of 150 min or more per week of moderate-intensity physical activity. In interpreting the results of RCTs, there is an insufficient body of evidence on the relationship between physical activity and cognitive health. However, studies indicated that moderate-intensity physical activity had a positive effect on cognitive health. Implications: Given the broad consensus of a dose-response relationship between aerobic activity and a variety of health outcomes, the RCT literature appears to have underestimated the benefit of physical activity for previously sedentary older adults because the prescribed dosages are not consistent with those recommended.