Circulating tumor cells (CTCs) are cancer cells that are present in the blood of patients with solid cancers and are shed from existing tumor lesions into the blood stream. The enumeration of CTCs has long been considered to hold great promise in guiding treatment decision-making in breast cancer patients. However, guidelines on how to use CTC enumeration in clinical decision-making in primary breast cancer and metastatic breast cancer are lacking. Here, we set out to review the most relevant literature to date, to ultimately come to general recommendations regarding the use of CTC enumeration in primary breast cancer and metastatic breast cancer.