The interest in posterior composite resins has grown rapidly during the last several years. Much of the interest has been initiated by a demand for esthetic dentistry. Some has developed as a result of a concern by some for mercury sensitivity. Despite their growing popularity, a number of major problems persist. Some of these deal with the restorative material, whereas others relate to the clinical techniques associated with their use. Before the clinician considers the use of composite resins in posterior teeth, he or she should be familiar with conditions that strongly influence clinical behavior.