The clinical diagnosis of secondary caries is by far the most common reason for replacement of restorations, but the scientific basis for the diagnosis is meager. The purpose of this article is to review the literature on secondary (recurrent) caries and present case reports to document the problems encountered in the clinical diagnosis of secondary caries. The literature on secondary caries was critically reviewed and subdivided into clinical diagnosis, location of secondary lesions, histopathology, microleakage, and microbiology. The case reports included restorations that were scheduled to be replaced because of secondary caries or stained margins of composite restorations. The lesions were photographed preoperatively and postoperatively. Based on the limited literature available, secondary caries appears to be a localized lesion similar or identical to primary caries. It is most often localized gingivally on restorations. Narrow gaps, crevices, ditches, and 'microleakage' do not lead to secondary caries, but wide voids may. Secondary caries is difficult to diagnose clinically. Consistency or hardness and discoloration of dentin and enamel are the best parameters. Secondary caries is the same as primary caries located at the margin of a restoration.