This book is about the behaviour of systems. Systems are important, for we interact with them all the time, and many of the actions we take are influenced by a system – for example, the system of performance measures in an organisation influences, often very strongly, how individuals within that organisation behave. Furthermore, sometimes we are involved in the design of systems, as is any manager contributing to the definition of what those performance measures might be. That manager will want to ensure that all the proposed performance measures will drive the ‘right’ behaviours rather than (inadvertently) encouraging dysfunctional ‘game playing’, and so anticipating how the performance measurement system will work in practice is a vital part of a wise design process.