Matrix metalloproteinases (MMPs) seem to play a dual role in dentistry. While several MMPs have an important role to play in developmental defects of teeth and in caries, some MMPs also seem to have a defensive role. The main organic component of tooth structure is collagen and MMPs that degrade collagen and the extra cellular matrix have been implicated in progression of dental caries. MMPs have also been shown to be active in pulpitis and studies have shown that they can be used as diagnostic markers of pulpal inflammation. This book reviews the role of MMPs in conservative dentistry and endodontics.