When a very young woman is diagnosed with breast cancer, she may face several threats to her future health and wellbeing. Most concerning, a young woman with breast cancer is more likely than an older woman to have an adverse prognosis. The differential in prognosis by age may reflect, in part, biological differences between breast cancer that develops in a younger compared with an older woman. Prognosis may, however, also be affected by suboptimal therapy, particularly endocrine therapy, in the youngest patients who are least likely to lose ovarian functioning as a result of systemic therapy. Because of the relative rarity of breast cancer in young women, large pooled analyses and multinational clinical trials are necessary to address the many controversies and improve therapy for younger patients. Late health and psychosocial effects of breast cancer in young women should also be considered in this vulnerable population.