The World Wide Web can be considered as a large distributed information system that provides access to shared data objects. As one of the most popular applications currently running on the Internet, the size of World Wide Web is of an exponential growth, which results in network congestion and server overloading. Web caching has been recognized as one of the effective schemes to alleviate the server bottleneck and reduce the network traffic, thereby minimize the user access latencies. In this paper, we first discribe the elements of a Web caching system and its desirable properties. Then, we survey the state-of-art techniques which have been used in Web caching systems. Finally, we discuss the research frontier in Web caching.