We propose constraints as the appropriate computational constructs for the design of agents with the task of selecting, merging and managing electronic information coming from such services as Internet access, digital libraries, E-mail, or on-line information repositories. Specifically, we introduce the framework of Constraint-Based Knowledge Brokers, which are concurrent agents that use so-called signed feature constraints to represent partially specified information and can flexibly cooperate in the management of distributed knowledge. We illustrate our approach by several examples, and we define application scenarios based on related technology such as Telescript and workflow management systems.