In England and Wales, national government has introduced a range of policies for dealing with the most dangerous offenders. These include new sentencing arrangements, new treatment programmes, and enhanced supervision of those released on licence. Policy makers needed to estimate the impact of this policy on the prison population and to consider how much treatment and community supervision capacity would be needed over time. They worked with a small SD consultancy to develop and build a model using ithink software, to enable a variety of scenarios to be tested, adopting a group model building approach. A number of staff received training in SD, including model building. The paper outlines the policy background, model structure and examples of scenarios. As well as being a practical application of SD to a sensitive area of public policy, the project is an example of what can be achieved within a relatively short intervention.