The application environments and connection with the Internet require that wireless ad hoc networks should support Quality-of-Service (QoS). In a wireless ad hoc network, supporting quality-of-service is very difficult because of the inherent characteristics of wireless channel and the frequent change of network topology. This paper comprehensively summarizes and deeply analyzes the researches of recent years on the QoS architecture, QoS routing, QoS signaling, and media access control (MAC) mechanism that supports service differentiation and resource reservation in wireless ad hoc networks. Based on the analysis, this paper deeply discusses the problems that must be resolved to support QoS in wireless ad hoc networks, and points out the direction to the research in the future.