Mesenchymal stem cells (MSCs) are a heterogeneous mix of stromal stem cells that can give rise to cells of mesodermal lineages, namely adipocytes, osteocytes and chondrocytes. They can home to sites of injury where they promote the repair and regeneration of damaged tissues. MSCs also home to sites of tumorigenesis, and as such, are utilized as efficient cellular vehicles for the delivery of anti-neoplastic therapeutics. Recently, MSCs within the tumor microenvironment have been shown to contribute to the desmoplastic reaction and to facilitate tumor formation and progression, sparking renewed interest in their pro-tumorigenic attributes and their roles as tumor stromal cells. Here, we describe the evidence linking MSCs to inflammatory processes and breast cancer development, and discuss their newly discovered physiological roles in the context of the tumor microenvironment.