A previous paper (Browne et al. 1981) identified the decision making problems associated with job shop control and presented an objective function for measuring its performance. Given that definition of job shop control and cost-based objective function, simulation modelling offers the best prospect of a solution of the problem of providing a management tool to aid in the decision making associated with job shop control. This paper provides a detailed analysis of the design of such a model. The model, written in the GASP fV simulation language and running on a CDC 7600 computer, is based on the job shop of a major machine tool manufacturer and has been validated against actual job shop performance.