The World Wide Web is growing and changing at an astonishing rate. For the information in the web to be useful, web information systems such as search engines have to keep up with the growth and change of the web. In this paper we study how web documents change. In particular, we study two important characteristics of web document change that are directly related to keeping web information systems up-to-date: the degree of the change and the clusteredness of the change. We analyze the evolution of web documents with respect to these two measures and discuss the implications for web information systems update.