Breast cancer is the most common cancer in the UK, with over 55,000 new cases diagnosed in the UK each year. Recent molecular analysis has revealed that the traditional, morphological categorisation has underestimated the variety of breast cancer types which exist. Many of these molecular subtypes vary in terms of their clinical behaviour (such as the tendency to metastasise) and their responses to therapy. Expanding on these initial studies offers great hope in terms of being able to predict tumour behaviour and patient outcome with greater accuracy, and also for the development of novel therapies targeted specifically against particular subtypes.