Wireless mesh networks (WMNs) have emerged as a key technology for next-generation wireless networking, due to independency of backbone networks. However, to make a WMN be all it can be, considerable research efforts are still needed. As the available MAC and routing protocols applied to WMNs do not have enough scalability as throughput drops significantly as the number of nodes or hops increases. Existing security schemes may be effective for certain types of attacks, but they lack a comprehensive mechanism to prevent attacks from different protocol layers. Similar problems exist in other networking protocols. Thus, existing communication protocols, ranging from routing, MAC, and physical layers, need to be revisited and enhanced. In some circumstances, new protocols need to be invented. Researchers have started to revisit the protocol design of existing wireless networks. This paper presents a detailed investigation of current research issues and challenges in wireless mesh networks.