For several years, we have been studying collaborative, dialogue-based interaction between humans and between humans and a computer system as part of the TRAINS project [Allen and Schubert, 1991]. The goal of the project is an intelligent planning assistant that is conversationally proficient in natural language and that will assist a human manager (the user) formulate and execute plans. The domain of discourse is a transportation world that includes trains, cities, connections between them, warehouses and factories, and different types of commodities, all of which are part of a simulated world in which the mutually-decided-upon plans are executed. Work in the TRAINS project consists of developing both formal models of the various tasks involved in collaborative dialogue and programs that implement these theories. Both will be described in this report.