ClinicalTrials.gov is a Web-based system intended for a diverse audience, including patients, family members and other members of the public. Throughout the system design and development process, our decisions have been driven by usability concerns. We first describe the overall design of the site, including the home page, which provides a site overview and rapid access to the information contained within it. Next we discuss the data presentation format which has been standardized in spite of data coming to us from many different sources. We provide a detailed description of the search and browse features that are intended to simplify the complexities of medical terminology and support information discovery. We conclude with a review of our evaluation activities and future plans.