To comply with legal and other standards, businesses and regulators are increasingly required to make decisions based on risk assessments of the potential effects of their activities on the environment. Atmospheric dispersion modelling is a cost-effective method, allowing various scenarios to be explored before expensive investment takes place. This guide offers advice on this environmental management tool. Unlike much of the previous literature, it doesn't focus excessively on the mathematical theory behind the modelling or on modelling for specific regulatory purposes. Instead, it offers an understanding of the background to the methodologies, providing exercises to develop the skills to carry these out and including examples of the use of commercially available models to enable the reader to assess the results of modelling for risk assessment.