Methodologies for building usable software systems have been introduced and refined over the past fifteen or so years under the discipline of Human-Computer Interaction (HCI). Much effort has been put into exploring cognitive models of human behavior as it relates to computer usage, and developing guidelines for screen layout and system dialogues. These are predictive endeavors whose purpose is to assist the software developer in the initial task analysis and system design. But, just as comprehensive functional requirements and a detailed design document do not by themselves guarantee that a programmer's final code will be correct, so up-front usability guidelines do not by themselves guarantee a usable end product. In both cases a distinct validation process is required. Usability testing is the process by which the human-computer interaction characteristics of a system are measured, and weaknesses are identified for correction. This article gives an overview of the usability testing process and describes a set of testing techniques the authors have used to evaluate the Bureau of Labor Statistics public access Web site and a joint BLS-Bureau of the Census Web site for the Current Population Survey. This work has been conducted largely for release over the Internet, but the techniques described are equally relevant to intranet testing.