We have developed a methodology for studying and analyzing the psychology of users performing ecologically valid WWW tasks. A user trace is a record of all significant states and events in the user-WWW interaction based on eye tracking data, application-level logs, and think-aloud protocols. A user-tracing architecture has been implemented for developing simulation models of user-WWW interaction and for comparing a simulation model (SNIF-ACT) against user-trace data. The user tracing architecture compares each action of the SNIF-ACT simulation directly against observed user actions. The model and architecture have been used to successfully match detailed user trace data from four users working on two tasks each.