Most cases of breast cancer are diagnosed in older adults. Older women have an increased risk for breast cancer–specific mortality and are at higher risk for treatment-associated morbidity than younger women. However, they are also less likely to be offered preventive care or adjuvant therapy for this disease. Major gaps in evidence exist regarding the optimal evaluation and treatment of older women with breast cancer because of significant underrepresentation in clinical trials. Chronologic age alone is an inadequate predictor of treatment tolerance and benefit in this heterogeneous population. Multiple issues uniquely associated with aging impact cancer care, including functional impairment, comorbidity, social support, cognitive function, psychological state, and financial stress. Applying geriatric principles and assessment to this older adult population would inform decision making by providing estimates of life expectancy and identifying individuals most vulnerable to morbidity. Ongoing research is seeking to identify which assessment tools can best predict outcomes in this population, and thus guide experts in tailoring treatments to maximize benefits in older adults with breast cancer.