Although breast cancer is rare and understudied in adults age 40 and younger, recent epidemiologic data show an increasing incidence of breast cancer among young women in the United States and ongoing inferior long-term outcomes. Given breast cancers arising at a young age are more likely to present at advanced stages and to have aggressive biology, multimodal treatments are often indicated. Elevated local recurrence risks and greater propensity for germline cancer predisposition mutations can impact local therapy choices. Recently, escalated systemic therapy regimens for triple-negative breast cancer incorporating immunotherapy, de-escalated anti-HER2 therapy, and emerging targeted agents, including CDK4/6 inhibitors and PARP inhibitors, for early-stage disease may be employed in younger and older patients alike, with some special considerations. Prognostic genomic signatures can spare low-risk young women with hormone receptor-positive breast cancer adjuvant chemotherapy, but management of intermediate-risk patients remains controversial. Ovarian function suppression and extended endocrine therapy are improving outcomes in hormone receptor-positive breast cancer, but treatment adherence is a particular problem for young patients. Young women may also face greater challenges in long-term survivorship, including impaired fertility, difficulties in psychosocial adjustment, and other treatment-related comorbidities. Consideration of these age-specific issues through dedicated multidisciplinary strategies is necessary for optimal care of young women with breast cancer.