The popularity of the World-Wide-Web has increased dramatically in the past few years. Web proxy servers have an important role in reducing server loads, network traffic, and client request latencies. This paper presents a detailed workload characterization study of a busy Web proxy server. The study aims in identifying the major characteristics which will improve modelling of Web proxy accessing. A set of log files is processed for workload characterization. Throughout the study, emphasis is given on identifying the criteria for a Web caching model. A statistical analysis, based on the previous criteria, is presented in order to characterize the major workload parameters. Results of this analysis are presented and the paper concludes with a discussion about workload characterization and content delivery issues.