Elderly persons, a rapidly growing population segment, have an increased incidence of cancer. The older cancer patient's clinical evaluation and treatment is influenced by conditions such as disabilities, comorbidity, and functional status, along with tumor type and stage. These conditions and other geriatric syndromes can be identified by comprehensive geriatric assessment to guide therapy and affect prognosis and quality of life. Comprehensive geriatric assessment involves the medical, functional, affective, social, spiritual, and environmental assessments. The medical assessment, which includes a nutrition, vision, hearing, continence, gait and balance, and cognition evaluation, can provide additional information to performance status and comorbidity. Although there are many assessment domains using several instruments, comprehensive geriatric assessment can be focused and efficient, especially with a multidisciplinary team of nurses, social workers, pharmacists, and other personnel. Comorbid illnesses may have complex interactions, with the underlying cancer influencing cancer diagnosis, disease course, treatment-related side effects, and mortality. Many instruments are available for comorbidity measurement, and retrospective studies in elderly cancer cohorts have shown comorbidity to influence survival. However, the ultimate aim would be to use comorbidity and comprehensive geriatric assessments prospectively in the older cancer patient to help predict the suitability and success of treatment with various antineoplastic modalities.