Among all the known differences between cancer and normal cells, it is only the genetic differences that unequivocally distinguish the former from the latter. It is therefore not surprising that recent therapeutic advances are based on agents that specifically target the products of the genes that are mutated in cancer cells. The ability to identify the patients most likely to benefit from such therapies is a natural outgrowth of these discoveries. Development of companion diagnostic tests for this identification is proceeding but should receive much more attention than it currently does. These tests can simplify the drug discovery process, make clinical trials more efficient and informative, and be used to individualize the therapy of cancer patients.