The monograph presents a review of modern ideas about the pathogenesis of breast cancer, including molecular risk factors of aggressive tumour growth in the breast and disorders occurring in the genetic apparatus of the cell. The main problems of individualisation of treatment are considered, the basics of personalised medicine and its importance in treatment are given. The authors of the monograph to improve the effectiveness of treatment of cancer patients with breast cancer before the start of treatment offer to carry out genetic testing, i.e. to identify genetic factors that determine or determine the response of the organism to the action of drugs.