The Nicholson–Bailey model was developed in the 1930s to describe the population dynamics of a coupled host-parasite (or predator-prey) system. It is named after Alexander John Nicholson and Victor Albert Bailey.
The model uses difference equations to describe the population growth of host-parasite populations. The model assumes that parasites search for hosts at random, and that both parasites and hosts are assumed to be distributed in a non-contiguous ("clumped") fashion in the environment.
In its original form, the model does not allow for stable host-parasite interactions. To add stability, the model has been extensively modified to add new elements of host and parasite biology. The model is closely related to the Lotka–Volterra model, which uses differential equations to describe stable host-parasite dynamics.