This chapter introduces opportunistic networks. Such networks support spontaneous interaction between mobile users carrying mobile devices with them. After having been presented with the motivation for this new type of network, the reader will learn the underlying concepts, including an opportunistic network definition. Next, this chapter discusses what makes opportunistic networks different to mobile peer-topeer networks and mobile ad hoc networks; two network types that are closely related. We present a number of applications with a focus on data dissemination. As a sequel to that, the chapter discusses human factors that are important for opportunistic networks, namely privacy preserving techniques and an incentive scheme. The chapter concludes with an overview of future research issues by naming a number of open and unsolved problems.