Why does it matter is animals cancer studies are worthwhile or worthless or someplace in between? The answers to this question is that regulation of exposure to chemicals, including the intermittent exposure to trace elements to which the general public is subject, is largely based on interpretations of animal cancer bioassays. If these tests are reasonably accurate in predicting the probability, sites, and severity of human cancers, then regulation of chemicals suspected of causing cancer (carcinogens) is on firm ground. But if these animal cancer tests are weak or worse, so that one cannot reasonably predict human cancers from them, then regulation rests on quicksand.