This article discusses the current understanding of the interactions between tumors and cells of the immune system, particularly at the early stages of carcinogenesis. A growing body of data suggests that these interactions help shape the eventual development of tumors. Inflammation is a common feature of several cancers, and the immune system can serve as a two-edged sword against cancer, capable of supporting and suppressing cancer. Data from human studies show that the immune system is capable of detecting the smallest expansions of transformed cells, well before the development of clinical cancer. These advances suggest a need to change the current emphasis for harnessing antitumor immunity from therapy to prevention of cancers.