The present issue of Computer Communications is dedicated to opportunistic networks (OppNets), which are an instance of the delay tolerant networking (DTN) paradigm. Opportunistic networks have emerged as one of the most interesting evolutions of legacy Mobile Ad Hoc Networks (MANETs). In MANETs a continuous end-to-end path had to be established before a message was exchanged, meaning that the sender and the receiver had to stay simultaneously connected to a common network. Unfortunately, in real mobile ad hoc networks, due to node mobility, devices that are turned off or run out of battery and the intrinsic wireless link instability, the connectivity is such that an end-to-end path between message source and destination might never exist. In order to address this challenging scenario, OppNets turn the obstacle, i.e., mobility, into an opportunity by allowing nodes to carry the messages with them while they move, until a next hop deemed suitable is encountered. This is the new store, carry, and forward paradigm. Opportunistic networks are sometimes also referred to as Pocket Switched Networks, hinting at the fact the user devices act as both routers and end users of the system.