Liquid biopsy, the examination of body fluids for certain biomarkersBiomarkers, is considered a promising future technology for the early detection and successful treatment of diseases. In addition to free circulating DNA (cfDNA)Circulating DNA (cfDNA), RNAsRNAs, cells, and exosomesCells and exosomes also carry information that can be used as biomarkers for analysis. In the field of tumor diagnostics, it has been shown that the number of tumor cells in the blood, so-called circulating tumor cells (CTCs), can be linked to the formation of metastases and provides information about the severity of the disease. In the future, not only the number but also the molecular analysis of these CTCs will additionally be used in the context of personalized medicine for the targeted therapyTargeted therapy of tumor diseasesTumor diseases. For analysis, CTCs must be enriched and isolated from the blood sample in a robust and sensitive manner. For this reason, a wide range of semiautomated and fully automated strategies that enable the isolation of rare tumor cells from liquid samples will be discussed in this chapter. It also explains how the automated isolation of circulating tumor cells fits into the field of “immunoassaysImmunoassays.”