This chapter describes the user-centered design approach we adopted in the development and evaluation of an adaptive Web site. The development of usable Web sites, offering easy and efficient services to heterogeneous users, is a hot topic and a challenging issue for adaptive hypermedia and human-computer interaction. User-centered design promises to facilitate this task by guiding system designers in making decisions, which take the user’s needs in serious account. Within a recent project funded by the Italian Public Administration, we developed a prototype information system supporting the online search of data about water resources. As the system was targeted to different types of users, including generic citizens and specialized technicians, we adopted a user-centered approach to identify their information needs and interaction requirements. Moreover, we applied query analysis techniques to identify further information needs and speed up the data retrieval activity. In this chapter, we describe the requirements analysis, the system design, and its evaluation.