During the early formation and growth of primary tumor (e.g., breast, colon, or prostate cancer), cells are shed from the primary tumor and then circulate through the bloodstream. Many of the major recent advances in targeted therapies have relied on the acquisition of tumor tissue via biopsy before initiation of therapy or after the onset of resistance. The advantage of physical properties is that they allow circulating tumor cells separation without labelling. Methods based on physical properties include density gradient centrifugation, filtration through special filters. In addition to using somatic point mutations as markers for the detection of tumor DNA, strategies to detect tumor-derived rearrangements and chromosomal copy number changes in the plasma of patients with cancer have been developed. Several studies have shown that metastatic cells might have unique characteristics that can differ from the bulk of cancer cells in the primary tumor currently used for stratification of patients to systemic therapy. In conclusion, the molecular and functional analysis of circulating tumor cells and circulating nucleic acids can be used as companion diagnostics to improve the stratification of therapies and to obtain insights into therapy-induced selection of cancer cells..