Although increasing age is the major risk factor for breast cancer incidence and mortality, when adjusted for disease stage, breast cancer mortality is similar among younger vs older patients. Importantly, about 90% of older women with breast cancer present with early-stage disease. The biologic characteristics of breast tumors in older patients suggest they would derive benefit from adjuvant therapy, particularly endocrine therapy, but older women are still frequently undertreated, resulting in poorer survival. Studies suggest that focusing on comorbidity rather than “chronologic age” as a surrogate for life-expectancy is a key aspect of adjuvant decision-making for older patients. Morbidity and mortality from cancer in vulnerable patients with poorer health can be accurately predicted by the Comprehensive Geriatric Assessment (CGA), which evaluates comorbidities, functional status, cognition, social support, psychological state, nutritional status, and polypharmacy. Use of the CGA and newer versions of this tool can lead to interventions that maintain function and improve quality of life in older patients with breast cancer.