Aromatase inhibition is an effective and rate-limiting step in estrogen production and in the treatment of advanced postmenopausal breast cancer. More than 3 decades of active research have already been put in its design, development and evaluation. The aromatase inhibitors can be classified either based on their mechanism of action, i.e. competitive aromatase inhibitors, which compete with the substrate androstenedione for non-covalent binding to the active site of the enzyme, irreversible inhibitors that bind to the active site of the enzyme covalently and mechanism-based or suicide inhibitors, which mimic the substrate and require catalytic amounts of the enzyme to be converted to the reactive intermediate resulting in inactivation of the enzyme. Aromatase inhibitors can also be classified as non-steroidal and steroidal types. The prototype nonsteroidal inhibitor aminoglutethimide provided the impetus for the development of related compounds while the steroidal inhibitors function as false substrates for aromatase with expected high selectivity. This review article focuses mainly on the steroidal aromatase inhibitors that have been developed to date, and which are classified on the basis of various modifications in the different rings (A,B,C,D) and position C-19 in the basic androstene nucleus. The second-generation drug 4-hydroxyandrostenedione (formestane), which is an excellent example of success achieved by carrying out modification in ring A, was introduced to clinical practice in 1990. Among the latest example is the orally active, third generation drug exemestane which is an irreversible inhibitor. The full potential of the new steroidal aromatase inhibitors is currently being investigated by many workers to evaluate their use in the management of breast cancer either directly or as an adjuvant to surgery in postmenopausal patients with early diagnosis.