Domain-oriented design environments are cooperative problem-solving systems that support designers in complex design tasks. In this paper we present the facilities and architecture of Argo, a domain-oriented design environment for software architecture. Argo's architecture is motivated by the desire to achieve reuse and extensibility of the design environment. It separates domain-neutral code from domain-oriented code, which is distributed among intelligent design materials as opposed to being centralized in the design environment. Argo's facilities are motivated by the observed cognitive needs of designers. These facilities extend previous work in design environments to support reflection-in-action, opportunistic design, and comprehension and problem-solving.