The identification and enumeration of tumor cells circulating in the peripheral blood (CTCs) of individuals with metastatic breast cancer has proven to be of independent prognostic value, and serial evaluations may be clinically useful for monitoring treatment response in patients actively undergoing therapy.Its role in earlier stage disease,prognostic stratification and potential utility as an endpoint in clinical trials has yet to be clearly defmed.Interrogation of these cells on a molecular level holds great promise to assist in therapy selection and individualization of treatment planning, as well as to provide information on differences in expression patterns between CTCs and the primary tumor that may significantly impact patient management.Given the growing recognition of their multiple potential clinical applications, a variety of new methodologies are under development to isolate and capture these cells, which are relatively few in number as compared to the surrounding normal blood components.It is likely that the evaluation of CTCs will increasingly be utilized in the assessment of some patients with breast cancer.