Many commentators have suggested that the ‘natural surveillance’ of public spaces can have important social consequences such as preventing or deterring crime. In this paper we introduce a model of natural surveillance in public space which uses visibility graph analysis. The model is applied to two study areas with very different urban morphologies: a traditional street network and a modern university campus. Levels of surveillance in the traditional street network are found to be much higher than in the university campus. It is suggested that the model can be used to test for the importance of natural surveillance as a risk-reduction factor in crime prevention through environmental design.