The law of mass-action as a paradigm for describing the contact rate of individuals in a population has been in widespread and heavy use in ecology and epidemiology for almost 100 years. The law roughly states that the rate at which individuals of two types, X and Y, meet is proportional to the product of the (spatial) densities of the respective subpopulations: xy.The law originates in the theory and practice of chemical reaction kinetics. The analogy between individuals meeting when moving around in space and molecules meeting when moving around in a gas or solution is intuitively pleasing. Moreover, the simplicity of the interaction term widens the possibilities to analytically study the behaviour of the systems of differential equations incorporating this description of the contact process. Both of these factors have undoubtedly been major determinants in creating the success of mass action as a modelling concept.