Breast cancer management has progressed immensely over the decades, but the disease is still a major source of morbidit y and mortalit y worldwide. Even with enhanced imaging detection and tissue biopsy capabilities, disease can progress on an ineffective treatment before additional information is obtained through standard methods of response evaluation, including the RECIST 1.1 cr iter ia, widely used for assessment of treatment response and benefit from therapy. 6 Circulating biomarkers have the potential to provide valuable insight into disease progression and response to therapy, and they can serve to identify actionable mutations and tumor characteristics that can direct therapy. These biomarkers can be collected at higher frequencies than imaging or tissue sampling, potentially allowing for more informed management. This review will evaluate the roles of circulating biomarkers in breast cancer, including the ser um mar kers Carcinoembryonic antigen CA15-3, CA27-29, HER2 ECD, and investigatory markers such as GP88; and the components of the liquid biopsy, including circulating tumor cells, cell free DNA/DNA methylation, circulating tumor DNA, and circulating microRNA.