It is generally accepted that nutritional behaviors constitute decisive components of human health. Phytochemicals (small, nonenergetic molecules of vegetal origin) are overall inhibitory on the expression of gene products promoting proliferation and resistance to apoptosis. On the contrary, phytochemicals stimulate the synthesis of adaptive proteins that favor resistance to cellular stress (detoxifying and antioxidant enzymes). They are effective modulators that act synergistically on membrane, cytoplasmic and nuclear enzymatic reactions to dampen cellular hyperproliferation and hyperactivity, reequilibrate metabolic activity and promote apoptosis of genetically unstable cells. Despite important gaps in our knowledge regarding how phytochemicals interfere with cellular function in vivo, effective chemopreventive measures have shown that phytochemicals can be utilized to prevent cancer, and possibly to treat cancer patients as well. We review how phytochemicals exert their beneficial effects at the cellular level.