Systems thinking is a way of describing and illustrating complex problems with many components and interrelationships in a way that can (a) be understood quickly by a wide range of experts; (b) be used to identify and isolate particular areas of the problem space to make them more tractable (using control theory, for example); and (c) be used to understand the potential consequences of any interventions. The overall aim of the paper is to apply and to promote the use of systems thinking to policymaking. To do this, the paper gives a background to systems theory and provides a simple practical example to show how it can be applied to the UK postal market, drawing out both the benefits and limitations of the approach. The paper then looks at how system theory could be extended into other areas, how it is useful for other national regulators to understand similarities and differences within their own markets, and how it could be applied to other regulated industries such as telecoms.