The increase in cancer incidence with increasing age is becoming more obvious and more important as the average age of the population increases. The close link between old age and cancer development is the result of three main factors: the substantial length of time required for carcinogenesis; the occurrence of age-related molecular changes that mimic carcinogenesis; and, changes in bodily environment that favour cancer progression, which is a consequence of increasing age. The clinical behaviour of common malignant diseases, eg, breast, ovarian, and lung cancers, lymphomas, and acute leukaemias, may change with age because of intrinsic variation of the neoplastic cells and the ability of the tumour host to support neoplastic growth. Therapeutic decisions should be based on an estimation of the patient’s life expectancy, and risks and benefits should be weighted up accordingly. A comprehensive geriatric assessment of function, comorbidity, cognition, depression, social support, nutrition, and polypharmacy, would allow interventions to be tailored to individual needs. In developed countries, the numbers of older people who develop cancer are increasing and many questions remain unanswered. These issues include: the causes of the association of cancer and ageing; the age-related differences in cancer biology; the goals of cancer treatment in the aged; and the effectiveness of cancer prevention. We review the biological and clinical interactions of cancer and ageing and discuss the skills and knowledge necessary for caring for older patients.