Wireless multi-hop networks have become an important part of many modern communication systems. Some of the earliest examples were military communication networks utilizing wireless relays in remote areas. More recently, many industries have used wireless multi-hop networks to create a multitude of fascinating tools and systems. Motivated by classical routing solutions in the Internet, conventional routing attempts to find a fixed path along which the packets are forwarded. Such fixed path schemes fail to take advantage of the broadcast nature and opportunities provided by the wireless medium, and result in unnecessary packet retransmissions. In opportunistic routing, decisions are made in an online manner by choosing the next relay based on the actual transmission outcomes as well as a rank ordering of neighboring nodes. Opportunistic Routing in Wireless Networks provides the motivation for opportunistic routing, and presents several different algorithms which achieve better performance, in most scenarios, than conventional shortest path routing. This comprehensive survey provides a starting point for all researchers and students wishing to learn the background and technical details of opportunistic routing.