SIGNIFICANCE: Among the most highly investigated theories of aging is the mitochondrial theory of aging. The basis of this theory includes a central role for altered or compromised mitochondrial function in the pathophysiologic declines associated with aging. In general, studies in various organisms, including nematodes, rodents, and humans, have largely upheld that aging is associated with mitochondrial dysfunction. However, results from a number of studies directly testing the mitochondrial theory of aging by modulating oxidant production or scavenging in vivo in rodents have generally been inconsistent with predictions of the theory. RECENT ADVANCES: Interestingly, electron transport chain mutations or deletions in invertebrates and mice that causes mitochondrial dysfunction paradoxically leads to enhanced longevity, further challenging the mitochondrial theory of aging. CRITICAL ISSUES: How can mitochondrial dysfunction contribute to lifespan extension in the mitochondrial mutants, and what does it mean for the mitochondrial theory of aging? FUTURE DIRECTIONS: It will be important to determine the potential mechanisms that lead to enhanced longevity in the mammalian mitochondrial mutants.