In many applications, it is of interest to understand how different patterns interact with each other. For instance, localised light pulses can be used to transmit information through nonlinear optical fibres: In this case, one wants to minimise mutual interaction of consecutive pulses in order to avoid transmission errors. The interaction between patterns can often be described by ordinary differential equations, where the variables correspond to the positions or centres of mass of individual patterns. Some of the mathematical issues are to give a rigorous analysis of interacting patterns and to provide a way to easily derive, for general partial differential equations, the reduced equations that govern their interaction.