As population ages, the number of older adults with cancer is increasing rapidly. Chronological age per se is a poor guide for an oncologist to determine tolerance to cancer treatment. Older adults have been underrepresented in cancer clinical trials, leading to paucity of guidelines to meet the treatment challenges in this population. To evaluate an older adult with cancer, oncologists must understand age-related changes and identify the subset of population who is vulnerable and at risk of cancer treatment toxicity. Comprehensive geriatric assessments focusing on functional status, multimorbidity, nutritional status, cognitive impairment, and psychosocial support help recognize heterogeneity among older adults, leading to individualized approaches toward cancer treatment. The treatment decisions need to be made in collaboration with the patient's values and preferences.