INTRODUCTION Breast cancer is a heterogeneous disease that encompasses several distinct entities with different biological characteristics and clinical behaviors. Currently, treatment decisions for metastatic breast cancer patients are made based on a small number of well-established molecular biomarkers, including the oestrogen receptor (ER), progesterone receptor (PR), and the human epidermal growth receptor 2 (HER2), as well as clinicopathological features. The biological importance of these established biomarkers has been reinforced over the last decade with the identiﬁ cation of discrete genetic subtypes of breast cancer with distinct prognoses (section “Established Biomarkers” and chap 4) ( 1 , 2 ). Advances in high throughput technologies including proteomics, expression array technologies, copy number measurements, and most recently next generation sequencing, will signiﬁ cantly increase the potential to identify a variety of biomarkers and assign single tumours to speciﬁ c molecular subgroups, even when small amounts of tumour tissue are available ( 3 ). Furthermore, the study of circulating biomarkers (section “Circulating Biomarkers”) offers the promise of analysing tumours comprehensively at the molecular level in “real-time” without subjecting patients to multiple clinical interventions to obtain tissue. This chapter will focus on recent advances in biomarker research, especially the use of circulating biomarkers, and will give some examples as to how these could impact the management of breast cancer patients.