In the past years, a subset of regulatory T cells (Tregs) expressing CD4, CD25 and the transcription factor FoxP3 has gained considerable attention as key regulators of T-cell tolerance and homeostasis (Sakaguchi, 2004). This population of T cells is specifically engaged in the maintenance of immune self-tolerance and the control of aberrant immune responses to foreign antigens. Remarkably, regulatory T cells have been implicated in tumor cell evasion of immune responses (Curiel et al., 2004; Zou, 2006) by suppressing T cell mediated antitumor immunity. The study of the signals that promote the differentiation of this suppressive population in the tumor microenvironment has become a central issue. Here we described a detailed method to in vitro differentiate Tregs using tumor cells conditioned media from mouse naïve T cells and to identify them based on their specifics markers (Dalotto-Moreno et al., 2013).