Wireless Mesh Networks (WMNs) using IEEE 802.11s or IEEE 802.16s allow the multi-hop Internet access to extend the network service coverage without connecting each access point to the wired network, which could significantly reduce the investment and maintenance cost. As other wireless networks, the secure authentication with access control is a paramount important issue to ensure the service of legal users while preventing a variety of security attacks. In this book chapter, we provide a comprehensive understanding on the authentication. We introduce IEEE 802.1x security framework and show its limitations when the Extensible Authentication Protocol (EAP) is directly applied to WMNs. The security attacks in the WMN are illustrated and more importantly, we discuss the authentication performance requirements imposed by the unique WMN properties. A number of secure access control and authentication approaches are illustrated by which to show the current progress in developing robust and high performance authentication protocols.