The speed of flow of patients through a surgical unit and the use of resources, such as theatre time and beds, can have a complex relationship, but one that can be analysed using simulation models. These models aim to mimic the day-to-day activities of admission, operation, transfer and discharge according to characteristics of the unit and patients being treated. “What-if” analysis can be undertaken by making changes to assumptions about length of stay, the ward capacities or the number and timing of theatre sessions. Coopers&Lybrand have developed and used simulation models in a number of studies to evaluate different planning options.