According to current guidelines, the additional biopsy of breast cancer metastases to analyze the receptor status for phenotype assessment is recommended. However, due to clinical difficulties in performing biopsies of metastatic lesions, the phenotype of the primary tumor most often determines the treatment decisions in metastatic breast cancer. Liquid biopsy allows the analysis of several circulating biomarkers like circulating tumor cells (CTCs) or circulating tumor DNA (ctDNA) in peripheral blood samples of cancer patients. Thus, it is an elegant and easily practicable technique that delivers information on the current disease status. Determination of the CTC phenotype regarding the hormone receptor and human epidermal growth factor receptor 2 (HER2) status might replace additional tissue biopsy for planning further therapy strategies. Liquid biopsy is a crucial step towards a more individualized cancer therapy. In contrast to the conventional concept of tissue biopsy, it offers an easy, less invasive acquisition of biomaterial. In addition, it allows multiple repetitions and real-time monitoring of metastasized disease in the clinical routine. However, the clinical utility of liquid biopsy still needs to be evaluated.