The construct of frailty is germane to that of aging, but a clinical definition of frailty is still wanted. In the geriatric literature, frailty has been conceived in two different ways. The first one is a threshold beyond which the functional reserve of a person is critically reduced and the tolerance of stress negligible. The second is as a progressive reduction of functional reserve due to a progressive accumulation of deficit. In this construct it may be hard to distinguish frailty from aging. Neither concept has at present a clear application in the management of older cancer patients. Studies are needed to establish whether the construct of frailty proposed by Fried et al. may be predictive of decreased cancer-independent survival and of decreased treatment tolerance in older cancer patients.