Many epidemiologic studies have demonstrated that younger women with breast cancer have a worse survival than older women, which may potentially be related to more aggressive tumor biology. Despite aggressive treatment, local and distant failure rates are higher. This review examines the studies that have investigated whether young age at diagnosis is associated with biologically more aggressive cancers for young women with invasive breast cancer, ductal carcinoma in situ, BRCA 1 and 2 mutations, and breast cancer during pregnancy. Young women with breast cancer are more likely to present with higher stage than their older counterparts. Their tumors are more likely to be estrogen receptor-negative, higher grade, and have increased LVI, Ki-67, and p53; yet most studies show no difference in HER-2/neu expression between younger and older women. Recent advances in molecular biology have shown that lymph node negative, estrogen receptor-positive young women are more likely than older women to have a higher Recurrence Scoretrade mark and therefore a worse prognosis. Molecular profiling has also revealed that young African-American women with breast cancer are more likely to have the more aggressive basal type of breast cancer, which may contribute to their worse prognosis compared to young white women.