General categories are identified which indicate possible sources of error in the process of using a computer model to predict the value of a state-variable. Such a state-variable might be, for example, temperature during fire growth in a compartment. It is not the intention to imply that these categories are the only possible ones. It is hoped, however, that this chapter will help to clarify the nature of error in the use of computer models and indicate areas for possible future work. The ideas are general and not restricted to fire models.