Clinical evidence supporting the use of aromatase inhibitors (AIs) in adjuvant therapy for hormone-sensitive early breast cancer (EBC) has grown rapidly over the past few years and is reviewed in this article. The results of two studies-the Arimidex, Tamoxifen, Alone or in Combination (ATAC) trial and the Breast International Group (BIG) 1-98 trial-support the use of AIs as primary adjuvant therapy for EBC, with significantly prolonged disease-free survival, time to recurrence, and time to distant recurrence for both anastrozole and letrozole over tamoxifen. Furthermore, anastrozole has an established beneficial risk:benefit ratio compared with tamoxifen with mature data over the full 5-year recommended treatment period. For women who have already received 2-3 years of tamoxifen, switching studies with anastrozole [the Italian Tamoxifen Anastrozole (ITA) trial and the Austrian Breast and Colorectal Cancer Study Group (ABCSG) 8/Arimidex-Nolvadex (ARNO 95) trial] and exemestane [the Intergroup Exemestane Study (IES)] have also shown that 5 years of primary adjuvant tamoxifen therapy is not optimal and that switching to an AI should be considered. Finally, for those women who have completed 5 years of tamoxifen, based on the results of the MA 17 trial, extended adjuvant treatment with letrozole should be considered. Although no sequencing data are available yet, current evidence suggests that an AI should be the adjuvant treatment of choice over tamoxifen, and anastrozole is the only AI with mature adjuvant data to date.