Opportunistic routing is an emerging technology that takes advantage of the broadcast nature of wireless networks. In opportunistic networks, mobile nodes communicate even if a connecting route doesn’t exist. In Traditional routing protocols fixed routes are selected for transmission and packets are forwarded for each hop. In dynamic wireless environment, transmission failures occur frequently due to path breaks and demands retransmissions. This unreliable transmission causes additional traffic in the network and waste of network resources and bandwidth. Opportunistic routing promises to be an efficient routing protocol to improve the performance of wireless networks providing increased network throughput and transmission reliability. This paper discusses the concept behind opportunistic routing, its classification, metrics used, and efficiency of these protocols.