The problem of making public health policy is the problem of bridging the gap between people who are concerned with good public health and politicians who make policy. This chapter looks at the forces that shape policies and the trajectories of public health in the UK since devolution. It looks first at what different UK governments can and cannot do in public health; it then looks at the major differences between the politics of England, Northern Ireland, Scotland and Wales and explains how they shape the kinds of public health policies that get made. The constitution of the United Kingdom – the politics and policies it makes possible – might be one of the most important factors shaping public health and public health policy, but like most other political institutions it was not designed with good public health policy making in mind.