This paper reviews secondary caries which is the major reason for failure of restorations and therefore worthy of attention if operative dentistry is to be cost effective. The basic premise of the review is that the secondary caries process is difficult to diagnose and cannot be permanently treated away by operative management. Preventive therapy, dependent on the patients' self-care, is essential. However, the quality of the restorative material and the skill with which it is handled are also relevant to the prevention of further failure.