Although the molecular and genetic determinants of most sporadic breast cancers remain unknown, increasing understanding of molecular and genetic events affecting breast carcinogenesis has provided information about the potential roles of specific biomarkers in tumour development and spread. It is now recognised that mutations of some tumour suppressor genes appear to play important early roles in the formation of some breast cancers. In addition, alterations in proto-oncogenes may contribute to the development of some breast cancers. The study of breast tumour biology at the molecular level has led to the development of targeted drug design, which provides a variety of agents targeted at specific molecules for the prevention, diagnosis and treatment of breast cancer. This review will describe the recognised molecular targets in breast cancer.