Opportunistic networks aim at building networks out of mobile devices carried by people, possibly without relying on any pre-existing infrastructure. They evolve the networking in Mobile Ad hoc Networks (MANET), which after more than ten years of research, still has not massively entered the mass market. One of the main reasons of this has been identified in the lack of realization and a practical approach to the design of MANET, where the features of mobile networks are masked by the routing (and transport) layer. Higher layers see an Internet-like network abstraction, where wireless networks' peculiarities, such as mobility of users, disconnection of nodes, network partitions, links' instability, are seen as exceptions. This often results in the design of MANET network stacks that are significantly complex and unstable. Opportunistic networks look at mobility, disconnections, partitions, etc. as features of the networks rather than exceptions. Actually, mobility is exploited as a way to bridge disconnected "clouds" of nodes and enable communication, rather than a drawback to be dealt with. More specifically, in opportunistic networking no assumption is made on the existence of a complete path between two nodes wishing to communicate. Source and destination nodes might never be connected to the same network, at the same time. Nevertheless, opportunistic networking techniques allow such nodes to exchange messages. By exploiting the store-carry-and-forward paradigm, intermediate nodes (between source and destination) store messages when no forwarding opportunity towards the final destination exists, and exploit any future contact opportunity with other mobile devices to bring the messages closer and closer to the destination. This approach to build self-organising infrastructure-less wireless networks turns out to be much more practical than the conventional MANET paradigm. Furthermore, opportunistic networking can combine the technology of the mobile devices with the social behavior of the mobile devices' carriers. Thus, despite the fact that opportunistic network research is still in its early stages, the opportunistic networking concept is nowadays exploited in a number of concrete applications, and very successful projects as the Haggle project (http://www.haggleproject.org).