Usually when a dentist talks about tooth preparation (also called a prep), he is referring to the process of reshaping a tooth (or teeth) for crowns. The preparation, in a sense, is like the foundation for a house; the finished crown will only be as good as its foundation allows. If not enough tooth is removed then, when materials are added back in construction, the crown will be "fat", and the color will not be correct, in the case of porcelain. I feel that under preparation is the biggest mistake made in preparing teeth for crowns and bridges. In addition, if the sides of the tooth preparation are not parallel then the crown or bridge will not stay on correctly.