Aromatase is an estrogen synthetase. Estrogens are female sex hormones involved in the development and growth of breast tumors. Elucidation of the structure-function relationship of aromatase has been of significant interest since its inhibitors have shown great promise in fighting breast cancer. Aromatase belongs to the cytochrome P450 family and forms an electron-transfer complex with its partner, NADPH-cytochrome P450 reductase. Because of the membrane-bound character and heme-binding instability, no crystal structure of aromatase has been reported so far. Much remains to be investigated, including the 3-dimensional structure of aromatase, interaction between aromatase and reductase, catalytic mechanism of estrogen synthesis by aromatase, and the binding mechanism of aromatase inhibitors. This review presents current knowledge about structural and functional characteristics of aromatase to address unsolved mysteries about this enzyme.