1. A method for simulating behaviour of first and second interrelated components within a system comprising: modelling behaviour of said first and second components using first and second functional specifications; simulating behaviour of said first and second components in predetermined circumstances by instantiating at least one first entity within a hierarchy of interrelated entities; and instantiating at least one further entity in response to the or each instantiated first entity, the or each further entity being selected by a simulation system on the basis of its hierarchical relationship with the at least one first entity.