Circulating Tumor Cells (CTCs) are tumor cells found in cancer patients’ peripheral blood. Enumeration of CTCs can provide prognosis information for cancer management (Cristofanilli et al., N Engl J Med 351(8):781–791, 2004; Cohen et al., J Clin Oncol 26(19):3213–3221, 2008; de Bono et al., Clin Cancer Res, 14(19):6302–6309, 2008; Poveda et al., Gynecol Oncol, 122(3):567–572, 2011). However, the technical hurdle for studying CTCs is their rare presence in blood, thus, isolating them is a non-trivial task. Two major categories of technologies have been developed in the past to isolate CTCs based on their biological expression of antigens (affinity-based capture) or based on their physical properties (non-affinity based capture). This chapter dedicates itself to the non-affinity based method for CTC capture. CTCs, as tumor cells, are inherently distinct from normal blood components. The chapter touches on the how these differences are reflected in their gene expression profiles, as well as their physical properties. We discuss how researchers utilized the unique biomechanical and electrical properties of CTCs to isolate them from enormous numbers of erythrocytes and leukocytes present in peripheral blood. We begin the chapter with technologies utilizing biomechanical properties (cell density, size, deformability) to isolate CTCs and then move on to discuss the development of dielectrophoresis (DEP) based CTC isolation, based on their distinct electrical properties.