The World Wide Web has become the largest hypertext system in existence, providing an extremely rich collection of information resources. Compared with conventional information sources, the Web is highly dynamic in the following four factors: size (i.e., the growing number of Web sites and pages), Web pages (page content and page existence), hyperlink structures and users' searching needs. As the most popular and important tools for finding information on the Web, Web search engines have to face many challenges arising from Web dynamics. This paper surveys the research issues on Web dynamics and discusses how search engines address the four factors of Web dynamics. We then briefly discuss the main issues and directions of future development of Web search engines.