It is widely appreciated that age is a powerful risk factor for cancer. This review considers two possible explanations for the exponential increase in most neoplasms in the elderly, arguing in favor of the idea that tumor progression proceeds at faster rates in older mice and older persons than in younger mice and younger persons. Three processes that alter the rate of aging, i.e., speciation, food restriction, and selective breeding, have parallel effects on cancer incidence. This suggests that the two processes are intimately connected. Elucidation of the mechanism(s) by which the aging process controls the rate of tumor development deserves high priority in experimental oncology. One such possible mechanism, the age-associated decline in immunologic defenses, is discussed at length in an attempt to rehabilitate the idea that a loss of immune function may contribute to the vulnerability of the elderly to cancer.