The numbers of migrating health workers have increased significantly over the past few decades and patterns of global migration have become more complex as increasing demand for health workers in the world’s wealthiest countries has resulted in large numbers of health workers migrating from lower-income countries to work in higher-income countries. Health worker migration is contributing to severe shortages of trained health workers in the lower-income countries and areas of greatest need, thereby weakening health systems that are already fragile in many countries, particularly in sub-Saharan Africa.