**1. Introduction**

Secondary caries, the lesion at the margin of a restoration, has been widely considered as the most important and common reason for restoration replacement, regardless of the restorative material type [Collins et al., 1998; Dahl and Eriksen, 1978; Deligeorgi et al., 2001; Friedl et al., 1995; Mjör, 2005; Mjör and Toffenetti, 2000;]. As it develops after the initial caries has been removed and replaced by a restoration, 'secondary caries', which is often referred to as 'recurrent caries' by practitioners in North America, is used more commonly in Europe[Mjör, 2005]. The Fédération Dentaire Internationale defined secondary caries as a 'positively diagnosed carious lesion, which occurs at the margins of an existing restoration' [Fédération Dentaire Internationale, 1962]. Then two kinds of lesion may exist adjacent to the restorations: secondary caries and residual caries (remaining caries). The latter one is residual demineralized tissue left, due to the failure of eliminating all infected dentine or/and enamel during the cavity preparation. Therefore it is very difficult for clinicians to make an accurate diagnosis of secondary caries and provide a clear terminology. Nowadays it is generally acknowledged that secondary caries or recurrent caries is a primary carious lesion of tooth at the margin of an existing restoration, which occurs after the restoration has been used for some time [Mjör and Toffenetti, 2000].

Due to its importance to the longevity of the restorations and human oral health, over the past few decades, multiple of studies have been conducted both in vivo and in vitro to understand and prevent secondary caries, including the etiology and histopathology of secondary caries, the detective and diagnostic methods of secondary caries, the relationship between microleakage and secondary caries, as well as the cariostatic effects of various restorative materials. The purpose of this chapter is to present a systematic and brief review of secondary caries in order to draw people's attention to this common but also complicated dental disease.
