Mammary stromal tissue has a major role in the control and regulation of physiological processes in the breast. Recently, the function of stroma in supporting the tumourigenic process as well as responding to the oncogenic lesion has become clearer. This review differs from the conventional view in that it focuses on and discusses the newly available evidence that points to the fact that mammary stroma has a significant contribution in actively generating transformed lesions and tumours. As such, the oncogenic signals can be dependent or independent of genetic mutations in mammary stromal cells. As a supportive and responsive agent in tumourigenesis, the stroma is induced by tumour cells to express critical signals that drive proliferation, angiogenesis, and motility while suppressing cell death. As an oncogenic agent in tumourigenesis, the stroma can provoke tumourigenicity in adjacent cells in the absence of pre-existing tumour cells leading to the acquisition of genomic changes. Investigating the mechanism by which the tumourigenic cues of the stroma facilitate the generation of malignant epithelial cells will provide invaluable insights into the oncogenic process.