The MDG period saw dramatic progress for many of the world’s poorest, and UK aid played a vital role in this accomplishment. Simultaneously, economic growth more than halved the number of Low-Income countries. However, more people living in poverty now reside in middle income countries than any other income classification, yet donors often rely on economic figures to determine eligibility for financial support. As economies grow, donor support may change, we call this change ‘transition’. Transition, if mishandled, can prove devastating to a country’s population, health system, and its economy. This report introduces RESULTS UK’s work on this topic and considers policy conclusions alongside a case study from Nigeria.