AbstractResearch into computer systems to aid simulation model formulation has led to the construction of a natural language understanding system (N.L.U.S.). A description and short history of N.L.U.S.s leads into a discussion of the syntactic and semantic problems which have to be met. A classification scheme for N.L.U.S.s is proposed, and the simulation problem formulator described in this context.The N.L.U.S. simulation model formulator takes as input a sentence in English which conforms to a prescribed set of sentence structures. These restricted sentence structures resolve syntactic and semantic problems. The way in which the system accepts these sentences and organizes data structures based on them is outlined, along with the supporting error-handling mechanisms and editing facilities. A simulation model example is used to illustrate the flavour of the system by reference to complete interactive session listings.