Agents that perform tasks on behalf of the user have great potential for assisting users in coping with large, complex, and dynamic information spaces such as digital libraries. This dissertation also presents AGS, an open architecture designed to provide agent services to participants of digital libraries. In AGS, specific user agent functionality is defined collaboratively by librarians, who are experts in the organization of digital library resources, and subscribed end users (or digital library patrons), who determine what library resources are of interest. Users delegate tasks to, visualize, and control agents in multiple libraries via a uniform interface. The design of AGS strongly fosters agent control by the user, provides extensible agent functionality, and utilizes readily available technologies in the construction of an environment that actively cooperates with the user in taking advantage of information resources. AGS enhances current views on services to be enabled by digital libraries and offers a much needed testbed for the investigation of issues in the development, deployment and use of agents as the basis for a human-computer interaction paradigm.