Beyond enumeration, CTC characterization is expected to help guide therapeutic selection for personalized care of cancer patients. Different approaches may be used to simultaneously identify multiple CTC-specific markers for biological characterization; yet awareness of associated pitfalls is also important. We have focused this chapter on molecular profiling of CTCs following enrichment. We describe the MagSweeper technology that was specifically developed to isolate live and highly purified CTCs for pooled or single cell or pooled cell molecular analyses or for CTC growth in vitro or in vivo. However, most of what is discussed will apply to any multiplex analysis of CTCs, irrespective of the enrichment method.