In this project, we explored the hypothesis that as socioeconomic development improves access to healthcare, education, employment and social protection, people with disabilities are at risk of being left out or left behind, creating a 'disability and development gap'. Our aim was to identify where the gap exists, see whether it widens as development proceeds and understand the mechanisms needed to close the gap. Over the course of this three-year project, the Leonard Cheshire Research Centre at University College London, working in conjunction with leading academic researchers at 5 African Universities and 4 national Disabled People's Organisations in four countries (Kenya, Sierra Leone, Uganda and Zambia) set out to better understand the relationship between disability and development in each country across four domains (education, health, labour markets and social protection). These countries were chosen as they demonstrate a range of socioeconomic stages of development. All have ratified the UN Convention on the Rights of Persons with Disabilities and have national legislation supporting the right of equal inclusion for people with disabilities. This mixed-methods research used a range of integrated components, including policy and secondary data analysis, a household survey of 4839 households (13,597 adults and 10,756 children) and more than 55 focus group discussions, 65 key informant interviews and 130 in-depth interviews across the four countries. The study found that a disability and development gap does exist. Adults and children with disabilities are at risk of being left behind in education, health care and social protection compared to their non-disabled peers. And they are at risk of being left behind compared to members of their same households as development progresses. Ongoing effective twin-track (disability-specific and disability-inclusive) interventions are needed to close the gap and ensure it remains closed.