This article reviews the principal modes of dental composite material degradation and relates them to the specific components of the composites themselves. Particular emphasis is placed on the selection of the monomer resins, the filler content, and the degree of monomer conversion after the clinical materials are cured. Loss of mechanical function and leaching of components from the composites are briefly described, while a more detailed description is provided of studies that have considered the chemical breakdown of materials by agents that are present in the oral cavity, or model the latter. Specific attention will be given to the hydrolysis process of monomer and composite components, i.e., the scission of condensation-type bonds (esters, ethers, amides, etc.) that make up the monomer resins, following reaction of the resins with water and salivary enzymes. A synopsis of enzyme types and their sources is outlined, along with a description of the work that supports their ability to attack and degrade specific types of monomer systems. The methods for the study of biodegradation effects are compared in terms of sensitivity and the information that they provide. The impact of biodegradation on the ultimate biocompatibility of current materials is discussed from the perspective of what is known to date and what remains to be studied. The findings of the past decade clearly indicate that there are many reasons to probe the issue of biochemical stability of composite resins in the oral cavity. The challenge will now be to have both industry and government agencies take a pro-active approach to fund research in this area, with the expectation that these studies will lead to a more concise definition of biocompatibility issues related to dental composites. In addition, the acquired information from such studies will generate the development of alternate polymeric chemistries and composite formulations that will require further investigation for use as the next generation of restorative materials with enhanced biostability.