1. A computer system comprising a program execution environment which is distributed between a server execution environment of a server apparatus and a client execution environment of a client apparatus, said client execution environment being operable to execute a plurality of client components, and said server execution environment being operable to execute a server component corresponding to each client component, wherein at least one client component is selected to be a primary client component, the or each primary client component being operable such that its behaviour and/or state is influenced by simulation information which is derived from the same source as simulation information which is operable to influence the behaviour and/or state of the corresponding server component, and wherein a client component is selected to be a primary client component in dependence on a property of an instance of program execution in the client execution environment.