The immune system effectively prevents cancer, whereas severe immunodepression increases its incidence. Cancer immunoprevention is a strategy based on the concept that enhancement of tumor immunity in healthy individuals reduces cancer risk. It can be viewed as a kind of chemoprevention. For cancer immunoprevention, the cancer universe can be neatly divided between tumors caused - directly or indirectly - by infectious agents and all other tumors. Immunoprevention of tumors caused by infectious agents is already implemented at the population level for hepatitis B virus (HBV)-related hepatocellular carcinoma and for tumors caused by human papillomaviruses (HPV), like cervical carcinoma. Now the challenge is to develop immunological strategies to prevent the bulk ( > 80%) of human tumor burden, unrelated to infections. Both vaccines against tumor antigens and immune modulators can prevent tumor onset in cancerprone mice. These studies outlined the target antigens and the molecular and cellular mechanisms of cancer immunoprevention: a) the best target antigens are surface molecules controlling tumor growth and progression (oncoantigens); b) combinations of potent vaccines and nonspecific stimuli (adjuvants) yield the strongest protection; c) immunoprevention must start early in the natural history of tumors, before key progression events like the onset of carcinoma in situ; d) lifetime protection requires repeated boosts, to maintain a strong and steady immune response; e) antibodies and helper, rather than cytotoxic, T cells mediate long-term protection from tumor onset; f) immunoprevention can be combined with chemoprevention. The development of agents like tamoxifen, which went from cancer therapy to chemoprevention, could be a model for the translation of cancer immunoprevention from mice to humans.