book, whose goal is to comprehensively review in ten During the past 30 years we have witnessed the emer chapters the fundamental knowledge on breast can gence of new disciplines whose contribution to the understanding of the biology of breast cancer have cer. The opening chapter links epidemiology, influ been enormous. Advances in the field of steroid hor ence of geographic and environmental exposures on mone nuclear receptors, which control the expression cancer incidence to the development of the organ and of genetic programs involved in cellular processes its role in cancer initiation. Central to this approach that are essential for normal and aberrant cell is the endocrine control of breast development, the growth, have been major contributors to the therapy relationship of cell proliferation with the presence of and prevention of hormone-dependent cancers. The steroid hormone receptors in the breast epithelium, and finally the importance of full differentiation as a discoveries of the association of hereditary breast cancers with mutations in the BRCAl and BRCA2 tool for protecting the breast from developing cancer. genes, and the concept that they essentially represent Reported are studies of the pathogenesis of breast a continuum of mutations with different degrees of cancer that have elucidated the site of origin of breast penetrance within given families, have added consid cancer and have been validated by in vitro experi erable understanding to the molecular basis of breast mentation.