Breast cancer is the most common type of cancer among women, and clinicians have long recognized its heterogeneity. Its detection and treatment in early stages allow reduction of its mortality. Despite the advances and new strategies for combining surgical, radiotherapy and chemotherapy options, the percentage of patients developing metastases and progressing to advanced stages remains high. Even though serum tumor markers have been used for the early diagnosis of metastases, their determination did not show an effective impact on survival. More reliable methods are needed to detect metastases earlier than with the common clinical methods and thus start treatment before overt relapse. Early indicators of response or resistance to treatment are also an issue in clinical practice.In contrast, detection of circulating tumor cells could be a useful tool in early detection of breast cancer, its relapse and response to systemic chemotherapy. Extremely sensitive techniques are available that are easily applied to peripheral blood samples, which might provide enormous research possibilities in this area.