Carcinogenesis can be visualized either as a multistep process (initiation, promotion, progression, invasion, and metastasis) or as a continuum of mutagenic and mitogenic events, with the contribution of epigenetic mechanisms. The exponential growth of the neoplastic mass explains the importance of secondary prevention (early diagnosis) and of tertiary prevention. Primary prevention, which was successful in controlling occupational cancers, aims at minimizing exposures to carcinogens in healthy subjects and at favoring the intake of chemopreventive agents with dietary and pharmacological agents. Besides chemical carcinogens, often in the form of complex mixtures, the workplace may involve exposures to physical agents, such as sunlight and artificial illumination systems delivering UV radiation, or to biological agents, such as chronic viral infections (HBV, HCV, and HIV) associated with cancers. A controversial issue is the occurrence of threshold doses for carcinogens in the workplace and the environment.