In recent years, the detection of cell-free DNA (cfDNA) in blood has become increasingly important. This circulating cfDNA in blood contains DNA fragments from healthy cells as well as from tumor cells. The analysis of this so-called liquid biopsy should make it possible to detect the development and progress of malignant processes at an early stage and with as little invasion as possible. In addition, patterns of miRNA, methylation and proteins can be detected in the blood as a part of liquid biopsies as well. In the future, it is conceivable that this form of liquid diagnostics will also be used for therapy monitoring and aftercare. Due to the high risk of cancer in people with cancer predisposition syndromes (CPS), this blood-based and less invasive form of cancer detection is particularly promising for CPS patients.