Duncan Sutherland is perpetually irritated by annoying pedestrians getting in his way. To understand the mathematics behind modelling pedestrian dynamics, he consulted google and found the following paper: Burstedde, C. et al. ’Simulation of pedestrian dynamics using a two-dimensional cellular automaton’, Physica A, (2001) 507-525, which he decided to discuss in a SUMS talk. This talk will introduce the basic concepts of cellular automata, and will discuss the model of Burstedde et al. A pedestrian is modelled as a particle which makes a transition between cells with some probability p_ij. The most interesting feature of Burstedde’s model is the concept of a floor field. The floor field modifies the probabilty p_ij. A static floor field is used to model topological features such as walls and doors, and also features which are attractive or repulsive to pedestrians, such as emergency exits or fires. A dynamic floor field changes with the pedestrian flow and is used to model long range interactions.