The immune system is the built-in host defense mechanism against infectious agents as well as cancer. Protective immunity against cancer was convincingly demonstrated in the 1940s with syngeneic animal models (JNCI 18:769-778, 1976; Cancer Immun 1:6, 2001). Since then, the last century's dream has been to effectively prevent and cure cancers by immunological means. This dream has slowly but surely become a reality (Nature 480:480-489, 2011). The successful examples of immunoprophylaxis and therapy against cancers include: (i) targeted therapy using monoclonal antibodies (Nat Rev Cancer 12:278-287, 2012); (ii) allogeneic hematopoietic stem cell transplantion to elicit graft-versus-cancer effect against a variety of hematopoietic malignancies (Blood 112:4371-4383, 2008); (iii) vaccination for preventing cancers with clear viral etiology such as hepatocellular carcinoma and cervical cancer (Cancer J Clin 57:7-28, 2007; NEJM 336:1855-1859, 1997); (iv) T cell checkpoint blockade against inhibitory pathways including targeting CTLA-4 and PD-1 inhibitory molecules for the treatment of melanoma and other solid tumors (NEJM 363:711-723, 2010; NEJM 366:2443-2454, 2012; NEJM 369:122-133, 2013; NEJM 366:2455-2465, 2012); (v) antigen-pulsed autologous dendritic cell vaccination against prostate cancer (NEJM 363:411-422, 2010); and (vi) the transfer of T cells including those genetically engineered with chimeric antigen receptors allowing targeting of B cell neoplasms (NEJM 365:725-733, 2011; NEJM 368:1509-1518, 2013; Blood 118:4817-4828, 2013; Sci Transl Med 5:177ra138, 2013).This article provides an overview on the exciting and expanding immunological arsenals against cancer, and discusses critical remaining unanswered questions of cancer immunology. The inherent specificity and memory of the adaptive immune response towards cancer will undoubtedly propel cancer immunotherapy to the forefront of cancer treatment in the immediate near future. Study of the fundamental mechanisms of the immune evasion of cancer shall also advance the field of immunology towards the development of effective immunotherapeutics against a wide spectrum of human diseases.