Dietary intake of soy is associated with a decreased risk of both hormone-dependent and hormone-independent cancers. It has been proposed that genistein, the predominant isoflavone in soy foods, is responsible for this effect. In this review, the potential mechanisms of action of genistein at the cellular level are critically examined to determine which are physiologically relevant. We concluded that (i) only those mechanisms requiring genistein concentrations below 5 μg/ml (18 μM) should be considered, and (ii) more emphasis should be placed on the effects of genistein on events in normal cells or those from the early stages of the cancer process.