Circulating tumor DNA (ctDNA) refers to DNA fragments released from cancer cells into the bloodstream. Clinical utility of ctDNA in breast cancer has been explored in both metastatic breast cancer (MBC) and early-stage breast cancer (EBC) settings. In MBC, ctDNA can detect therapeutically targetable genomic alterations and has shown great potential in predicting treatment response or resistance. Accumulating data suggest that ctDNA might also have prognostic value in MBC. In EBC, emerging data have shown ctDNA's predictive and/or prognostic value in both neoadjuvant and adjuvant settings. Minimal residual disease (MRD) detection via ctDNA to detect clinical recurrence after curative therapy is a rapidly advancing field. In this review, we discuss the existing and emerging data regarding ctDNA utility in both MBC and EBC settings.