Reversible data hiding (RDH), in which the original cover medium can be exactly extracted from the marked content, has attracted considerable interests from the information hiding community. In this paper, current RDH techniques are first reviewed by introducing the following five types of schemes: early lossless-compression-based schemes, expansion-based schemes, recent lossless-compression-based schemes, content-adaptive schemes, integer-transform-based schemes. Then, by proposing some unsolved problems, the future prospects of RDH are also discussed.