Breast cancer is the most common cancer among women and about 80% of breast cancers express hormone receptors. Tamoxifen has been the most important form of adjuvant endocrine therapy over the last 25 years. The third generation aromatase inhibitors (AIs) are a new class of drugs challenging the central role of tamoxifen as adjuvant endocrine treatment in postmenopausal women with hormone receptor-positive breast cancer. Their effectiveness has been demonstrated in first line therapy as well in neoadjuvant setting with a statistically significant superiority over tamoxifen. Here we considered the role of adjuvant AIs in early stage breast cancer with an analysis reviewing the main adjuvant trials. We considered efficacy, side effects, optimal timing, duration of the therapy and whether specific subgroups may achieve particular benefit. In conclusion the upfront use of adjuvant anastrozole or letrozole is superior to tamoxifen with a good relative toxicity profile. Tamoxifen will continue to have a role where recurrence risk is low or if AI is poorly tolerated. Issues including the timing of administration (up-front or sequential), the duration of the therapy and the role of biomarkers such as PgR and HER2 in optimal selection remain unresolved.