Aromatase is a key enzyme responsible for in vivo estrogen biosynthesis.  Inhibition of the activity of the aromatase has become an alterative way for treatment of breast cancer.  In this review, the structure and catalytic mechanism of the aromatase is briefly introduced followed by thorough review of the progress in the study of the steroidal and non-steroidal aromatase inhibitors.  This review is focused on the natural compounds that exhibit the aromatase inhibition, which include flavonoids, xanthones, coumarins, and sesquiterpenes.  The structure-activity relationship of these compounds is also discussed.