Tamoxifen, a selective estrogen receptor modulator (SERM), has been used for many decades as the "gold standard" adjuvant treatment for patients with hormone-receptor-positive early breast cancer. This drug, when administered for 5 years, reduces the risk for recurrence, contralateral breast cancer (BC) and death. The optimal duration of tamoxifen in the adjuvant setting has not been established yet, but it has been demonstrated that 5 years are better than shorter treatment while it is still unclear if a prolongation of the treatment for more than 5 years is worthwhile. In the last few years, third generation aromatase inhibitors (AIs), either steroidal (exemestane) or non-steroidal (anastrozole, letrozole), have shown to be an effective alternative to tamoxifen in postmenopausal patients with BC regardless of its stage. These agents act by blocking the aromatase enzyme which converts androgens into estrogens. The goal of this article was to review the results of recent randomized trials comparing AIs to tamoxifen in postmenopausal women in the adjuvant setting. Two strategies have been utilized: a direct upfront comparison in which both tamoxifen and AIs were given for 5 years or an early switch in which AIs were administered after 2-3 years of tamoxifen for 3-2 years or vice versa. Both strategies have shown a superiority of AIs over tamoxifen and a different safety profile but, the optimal treatment modality has yet to be defined. Moreover, in an attempt to further reduce patients' risk of recurrence after the administration of tamoxifen for 5 years, three trials have evaluated the role of prolonging the adjuvant treatment with AIs for 5 more years in comparison to placebo (late switch). A significant improvement of disease-free survival and of overall survival in the subgroup of node-positive patients, at least in one trial, has been observed with AIs. Despite these important results several unanswered questions remain and the results of ongoing trials will hopefully clarify some of them.