It is now well established that the immune system can control neoplastic development and growth in a process termed immunosurveillance. A link between host immunosurveillance and neoplastic progression is revealed in cases where the immune response becomes compromised due to genetic or other pathological conditions, resulting in a substantially increased incidence and rate of spontaneous tumour formation in both preclinical animal models and patients. It has also been demonstrated in tumour-bearing hosts that the tumorigenic process itself can promote a state of immunosuppression that, in turn, facilitates neoplastic progression. The ability of neoplastic populations to induce a hostile microenvironment through both cell contact-dependent and -independent immunosuppressive networks is a significant barrier to effective cell-mediated immunity and immunotherapy. Thus, a competent immune system is integral for the control of neoplastic disease, and dissecting the plethora of tumour escape mechanisms that disrupt this essential host defense capability is integral for the development of effective immunotherapeutic paradigms.