During the past few years, the intensified detection of small (mammary) carcinomas causes an increase in the number of mammary cancers. Cancer of the mammary tissues has an almost individually unpredictable behavior and aggressiveness. Therefore, a better insight in the molecular biological defects, which are responsible for initiation and progressive aggressiveness of mammary cancer, is necessary. Proteomics are an alternative to identify proteins which initiate carcinogenesis and can be useful to predict cancer prognosis. Today, the most commonly used technique for large-scale protein identification in clinical samples is two-dimensional electrophoresis (2-DE) in combination with image analysis and MS. Using these techniques, qualitative and quantitative information can be achieved regarding protein forms and post-translational modifications. In the following article we review proteomic techniques that are now commonly used in order to elucidate the role of proteins in breast cancer.