A method for transparently monitoring and/or modifying web browsing activities over an entire computer network does not require modification of client software and can be implemented on a single server. By rerouting HTTP requests through a centralized gateway server, the sequence and timing of URLs accessed by individual clients are recorded, providing the full information required to recreate a user session. The client HTTP requests are rerouted through the gateway server by modifying URLs within HTML documents. For each document sent to the client, any original URL which points to another server is modified so that it points to a CGI script program on the gateway. The gateway server then fetches the requested document from the other server, modifies the URLs within the document, and passes it on to the client. Thus, subsequent requests from the client will automatically be directed through the gateway server. The method permits tracking individual web sessions and provides valuable information about user behavior. This information can be used for general purpose evaluation of web-based user interfaces to information systems. The technique can also be used to modify a browsing experience by layering or stripping hypertext content or functionality.