Secondary caries is reported as one of the most important reasons for replacing restorations. The patient's general caries risk plays an important role in the development of secondary caries. The connection, at the patient level, between various factors, the risk of caries and restoration factors, such as the presence of a gap and the kind of restoration material, deserves additional investigation. The minimum gap width in which secondary caries can develop is most likely less than 100 µm and could depend on the patient's caries risk. Composite restorations seem more susceptible to secondary caries than amalgam restorations in patients with high caries risk. So far, there is no unambiguous explanation for this difference. The prevention of secondary caries is comparable to that of primary caries and stresses the importance of fluoride, diet and oral hygiene.