Circulating tumor cells (CTCs) can be detected in all solid tumors and are shed into the blood stream early on. Since they can be rare (e.g. 1 CTC in 10 billion blood cells) and quite heterogenous (reflecting the significant heterogeneity of the cells within a tumor of the same patient), it is a challenge to detect and analyze all of them in a given blood sample. CTCs have the advantage that the information (protein, RNA, DNA) of the different CTC-phenotypes are not mixed with others but are contained. Therefore, changes within the mixture of phenotypes during therapy can be easier detected.