This toolkit is designed to give development practitioners the know-how they need to understand people's behaviours and to help people to make positive changes in their lives. It highlights how behaviour change is key to addressing many development issues such as child mortality, climate change, gender inequality, and poor governance, and offers practical and easy-to-read guidance on how to go about developing a behaviour change intervention. It is based on insights gained from decades of research and the practical experience of development workers and social scientists into what determines how people behave.