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Abstract The explosion of WWW traffic necessitates an accurate picture of WWW use, and in particular requires a good understanding of client requests for WWW documents. To address this need, we have collected traces of actual executions of NCSA Mosaic, reflecting over half a million user requests for WWW documents. In this paper we present a descriptive statistical summary of the traces we collected, which identifies a number of trends and reference patterns in WWW use. In particular, we show that many characteristics of WWW use can be modelled using power-law distributions, including the distribution of document sizes, the popularity of documents as a function of size, the distribution of user requests for documents, and the number of references to documents as a function of their overall rank in popularity (Zipf''s law). In addition, we show how the power-law distributions derived from our traces can be used to guide system designers interested in caching WWW documents. --- Our client-based traces are available via FTP from http://www.cs.bu.edu/techreports/1995-010-www-client-traces.tar.gz http://www.cs.bu.edu/techreports/1995-010-www-client-traces.a.tar.gz