Abstract: Opportunistic networks (OppNets) are one of the evolutionary mobile ad hoc networks whose communication links often suffer from disruption, while their totally different networking paradigm attracts extensive attentions from both researchers and developers. The message in OppNets is transferred only on the occasional encountering of pair-wise mobile wireless nodes, and the routing paradigm is referred to as “store-carry-and-forward” transmission characteristics to implement network communication. In such networks, continuous end-to-end connectivity may not be possible. Due to unique features of high mobility of nodes, frequent link variation and long communication delays, many opportunistic forwarding protocols present major security issues, the design of OppNets faces serious challenges such as how to effectively protect data confidentiality and integrity and how to ensure routing security, privacy and node authentication and incentive cooperation. In other words, systematic research on OppNets is still open and far from a widely-used practical system. In this paper, it first systematically describes the security threats and requirements in OppNets; then elaborates the popular research problems including secure routing, privacy protection, node authentication and incentive cooperation mechanisms in opportunistic networks; and then various security and trust schemes are comprehensively analyzed and compared. Finally it concludes and gives the future research directions.