Editor's Summary: California's Proposition 65 is by now a well-known regulatory tool for warning consumers about the potentially toxic components of products they consume or to which they are exposed. Rechtschaffen and Williams argue that while Proposition 65 has been subject to some abuses, it has also brought about important reductions in exposures to toxic substances. To make their point, they examine product reformulations caused by enforcement of Proposition 65's warning requirements over the past five years.