Estrogens play an important role in the development and progression of several types of cancers. The synthesis of estrogens occurs in almost all tissues of the body in addition to the gonads. The enzyme aromatase (CYP19A1) encoded by CYP19A1 gene is involved in the synthesis of estrogens. Genetic variations in CYP19A1 gene influence both the structure-function relationship of the enzyme and the rate of its synthesis. Extensive studies have reported different types of polymorphisms in the CYP19A1 gene and have shown that the polymorphisms, depending on their location in the gene, have different effects on the function and activity of the gene product. Association studies have been conducted and have led to the realization that interpopulation differences are widespread. Not only do polymorphic forms exert different effects on the development of different cancers, due possibly to the influence of other genetic variations, environmental, metabolic, and epigenetic factors, but also are important as they lead to the interindividual differences seen during treatment of the cancer state. This chapter covers important aspects of the aromatase function, the CYP19A1 gene structure, polymorphisms identified in the gene, different cancers and associated polymorphisms, and the role of the polymorphic forms in affecting the treatment strategies.