In the United States, three third-generation aromatase inhibitors are available commercially: anastrozole, letrozole, and exemestane. Anastrozole and letrozole are nonsteroidal agents, whereas exemestane is a steroid. The three agents differ in terms of structure and metabolic products and in the degree to which they suppress aromatase activity. The clinical significance of these differences is unclear. All three of the agents have been found to be equivalent or superior to megesterol acetate as a second-line therapy for metastatic breast cancer. In the first-line setting, large Phase III trials have demonstrated that anastrozole and letrozole are equivalent or superior to tamoxifen in women with metastatic disease. Multiple trials with widely varying study designs have been launched in the adjuvant setting comparing the aromatase inhibitors to tamoxifen. Early results from the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial suggest a small but statistically significant improvement in disease-free survival for anastrozole compared with tamoxifen, but further follow-up is needed. This article explores the efficacy and tolerability of the aromatase inhibitors in both the metastatic and the adjuvant settings.