Incidence of breast cancer in patients aged < 20 years has been estimated to be 0.1 per 100,000 women. Reported incidences are 1.4 for women aged 20-24 years, 8.1 for women aged 25-29 years, and 24.8 for women aged 30-34 years. Younger patients have been found to have a more aggressive presentation of disease at diagnosis, which is associated with dire prognoses compared with those in premenopausal older patients. Several biologic features might explain the more aggressive behavior of breast cancer in younger patients: higher grade and higher expression of Ki67, higher occurrence of vessel invasion, and less expression of estrogen and progesterone receptors. Choice of adjuvant therapies for women aged <35 years with breast cancer is based on data derived from trials on cohorts of older patients. On average, the effect of chemotherapy for premenopausal patients is substantial: recent evidence suggested that very young women with endocrine-responsive tumors had a higher risk of relapse than older premenopausal patients with similar tumors. This was not the case for patients with endocrine-nonresponsive tumors, for which effects of chemotherapy were similar across ages. Very young women with this disease are faced with personal, family, professional, and quality-of-life issues that further complicate the phase of treatment decision-making. The development of more effective therapies for very young women with breast cancer requires tailored treatment investigations and research focused on issues specific to these patients.