Circulating tumor cells (CTCs) have been extensively studied in breast cancer (BC), with large studies establishing CTCs as a robust prognostic biomarker in early and metastatic breast cancer (MBC). Several phase II and phase III trials have investigated the clinical utility of CTCs in BC. Here, we outline the current landscape for the use of CTCs in the clinic at different stages of BC, focusing first on early BC, then on MBC, with a particular focus on interventional clinical trials based on CTCs.