Sustainability and Transformation Plans (STPs) have been developed by NHS and local government leaders in 44 parts of England. The plans offer a chance for health and social care leaders to work together to improve care and manage limited resources. But will they succeed where other initiatives have failed? Despite the importance of STPs for the NHS and the public, little is known about the process of developing the plans and how the initiative has worked in practice. Based on a series of interviews with senior NHS and local government leaders which took place throughout 2016, this report looks at how STPs have been developed in four parts of the country.