The National Health Service (NHS) in England is in a state of transition as the government pushes forward a programme of significant reform. If the government achieves its stated objectives, the NHS will be transformed from a state-owned commissioning and provision system to one in which care is delivered by a diverse range of largely autonomous public and private organisations within a health care market. With this market now developing rapidly, the question of how best to regulate it becomes ever more pressing. Regulation is intended to guard against various 'market failures', such as the development of monopolies or quality deficiencies. So far, the objectives and architecture of NHS regulation are under-developed and the operation of (and importantly the limits to) the market are still being defined. The existing regulatory system is, therefore, unlikely to be fit for purpose as the market matures. A number of important issues remain unresolved - what should be regulated, by whom and how? [Introduction].