Antimutagens and anticarcinogens are natural or synthetic substances able to inhibit or to reduce spontaneous or induced DNA alteration. These inhibitors act at different levels from the penetration of the xenobiotic to the expression of the mutation and cancer. They act in extracellular and intracellular levels; they react directly with mutagens or on the processes of their activation. Antimutagens and anticarcinogens also act on DNA-repair process.