Dietary restriction (DR), typically a 20%-40% reduction in ad libitum or "normal" nutritional energy intake, has been reported to extend life span in diverse organisms, including yeast, nematodes, spiders, fruit flies, mice, rats, and rhesus monkeys. The magnitude of the life span enhancement appears to diminish with increasing organismal complexity. However, the extent of life span extension has been notoriously inconsistent, especially in mammals. Recently, Mattison et al. reported that DR does not extend life span in rhesus monkeys in contrast to earlier work of Colman et al. Examination of these papers identifies multiple potential confounding factors. Among these are the varied genetic backgrounds and composition of the "normal" and DR diets. In monkeys, the correlation of DR with increased health span is stronger than that seen with life span and indeed may be separable. Recent mechanistic studies in Drosophila implicate non-genetic co-factors such as level of physical activity and muscular fatty acid metabolism in the benefits of DR. These results should be followed up in mammals. Perhaps levels of physical activity among the cohorts of rhesus monkeys contribute to inconsistent DR effects. To understand the maximum potential benefits from DR requires differentiating fundamental effects on aging at the cellular and molecular levels from suppression of age-associated diseases, such as cancer. To that end, it is important that investigators carefully evaluate the effects of DR on biomarkers of molecular aging, such as mutation rate and epigenomic alterations. Several short-term studies show that humans may benefit from DR in as little as 6 months, by achieving lowered fasting insulin levels and improved cardiovascular health. Optimized health span engineering will require a much deeper understanding of DR.