There is usually a point during the design of fire safety systems for buildings or during a fire investigation when a question is asked that involves knowing how much there will be of something, how long it will take for something to happen or where it will happen. To answer these questions, some form of calculation is required, which will involve using a fire model. Fire models cover a spectrum of complexity ranging from simple equations to detailed computer simulations. Knowing which method is appropriate for a particular problem depends on knowing the capabilities of each approach and its limitations. This Information Paper discusses some of the problems that may be encountered and suggests some tips for effectively presenting the results. The paper focuses in particular on the practical use of numerical models. (A4, 8pp) Contents Introduction Fire models - 'Simple models' - Zone/network models - Computational fluid dynamics models - Thermal models - Integrated models Capabilities and limitations of fire models Validation and verification Practical use Presenting results - Oversimplification - Iso-surfaces are not 'smoke' or flames - It's behind you! - Visibility - Perception The future Box 1: Tips for good images References