Circulating tumor DNA (ctDNA) evaluates cellular components of cancer cells in plasma. Breast cancer (BC) is the most frequent one. Studying ctDNA is a non-invasive method to assess BC. Patients with BC have higher levels of ctDNA than those without. High ctDNA levels are more frequent in patients with metastatic disease with high tumor burden and with more aggressive disease, such as those with triple-negative phenotype and HER2+. ctDNA allows the selection of target treatment for metastatic disease. Elimination of ctDNA during neoadjuvant chemotherapy in early BC is associated with higher complete response rates and longer survival and also predicts relapses well in advance.