Several hormones are well known for their role in tumorigenesis. Among them estrogen is the best characterized hormone. In particular, stromal tissue-produced estrogen plays a key role in breast tumor development and progression, highlighting the importance of communications between stromal tissue and tumor cells in the tumor microenvironment. In this review, we update our current understanding of stroma-mediated expression of hormones (estrogen) and their role in tumorigenesis, focusing on how aromatase produced in stromal tissue affects tumor cell growth. We also briefly touch on hormone therapy involving selective estrogen receptor modulators and aromatase inhibitors.