Effective communication between academics and policy makers plays an important role in informing political decision making and creating impact for researchers. Policy briefs are short evidence summaries written by researchers to inform the development or implementation of policy. This guide has been developed to support researchers to write effective policy briefs. It is jointly produced by the NIHR Policy Research Unit in Behavioural Science (BehSciPRU) and the UCL Centre for Behaviour Change (CBC). It has been written in consultation with policy advisers and synthesises current evidence and expert opinion on what makes an effective policy brief. It is for any researcher who wishes to increase the impact of their work by activity that may influence the process of policy formation, implementation or evaluation. Whilst the guide has been written primarily for a UK audience, it is hoped that it will be useful to researchers in other countries.