Tooth decay, known formally as dental caries, has been a serious health problem for all nations since time immemorial. For centuries, tooth decay was thought to be the handiwork of an elusive and, in some cultures, evil tooth worm that gnawed holes into the white, highly mineralized enamel and left all those in its wake in pain. But superstition has yielded to science and its explanation that certain oral bacteria discharge mineral-eroding acid onto the enamel, starting the gradual process of decay. Over the last several decades, dental researchers have made tremendous progress in defining and learning to thwart the decay process. This work has involved the three-pronged strategy of discovery, innovation, and prevention - and produced one of the major public health success stories of the 20th century.