Book excerpt: This book provides a general overview of the challenges of economic development for the five billion people living in developing countries. While they constitute over 80 percent of the world's population, they account for only 40% of the world's output, and are home to 2.6 billion people living on less than $2.00 per day. Thinking on economic development has shifted over time. Early theories that stressed capital formation and a heavy reliance on the public sector proved inadequate. Gradually, economists began to see that development was a complex, multifaceted problem that combined economic issues with problems of poverty and income distribution, insititution building and governance. While there have been many failures, there have also been many successes. Countries such as China, Chile, Ghana, and Korea demonstrate that good policies and strong institutions can result in remarkable progress. However, many poor countries, particularly those in Africa continue to lag behind. Closing this gap remains a major challenge for the world, particularly as the growing population and output of developing countries accelerate tensions in such areas as trade, immigration and financial flows.