This publication focuses on the subject of signal transduction and its role in the pathogenesis and progression of human breast cancer. Within the past 10 years, the explosion in our knowledge of these processes has significantly altered our understanding of breast cancer, and resulted in the first receptor-based therapies. This volume examines a variety of relevant signalling pathways in mammary epithelium and explores their mechanisms and functional contribution to the evolution of this disease. The articles herein explore receptor-based signalling that occurs both at the cell surface and within the nucleus that controls the gene expression required for the breast cancer phenotype. Of course, any good series of reviews should open up more questions than they answer, and the current collection of review provides no exception. It is our desire that the discussions engendered by the reviews herein stimulate additional research on these topics. Clearly, further mechanistic understanding of these underpinnings of breast neoplasia has been and will be the wellspring for novel therapies of this disease.