The role of an agreed, shared task model as an intermediate representation of a design/decision task upon which (1) negotiation between an expert user and a knowledge engineer, and (2) interaction between an expert user and an intelligent design/decision support system are based, is the focus of the research presented. A declarative compositional approach to user-centered system design (within the DESIRE framework) is presented and illustrated on the basis of a shared task model for the design of environmental policy.