Myeloid derived suppressor cells (MDSC) are heterogeneous cell population consisting of myeloid progenitor cells and immature myeloid cells. These cells have essential immunoregulatory role in tumor bearing hosts and under different inflammatory conditions. No specific marker has been described to identify MDSC, which leaves their suppressor activity as their only hallmark function. In this review, we discuss the current in vivo and in vitro developed assays for elucidation of MDSC function and describe the discrepancies between murine and human MDSC in regard to their suppressor function. We also discuss antigen specificity of MDSC function and approaches to determine the effector function of these cells in vivo. Finally, we summarize different approaches currently being employed to target MDSC with the aim to enhance immune based therapies.