During the last 50 years, governments have implemented a wide range of slum upgrading projects and programs of varying scale and scope and with different levels of impact. The lessons learned from these experiences and the knowledge developed by UN-Habitat demonstrate that technically slum upgrading is an easy and straightforward urban regeneration intervention – there is sufficient experience, knowledge, skills, and know-how to be drawn upon internationally. But despite the wealth of knowledge and experience available in the world, the growth of slums and the multiplication of informal settlements are only getting worse, particularly in parts of Asia, Sub-Saharan Africa, and parts of Latin America. This is undermining the ability of cities to generate wealth, prosperity, economic growth, and human development. Considering the large scale of slums, their consolidation, and the number of people they house, ignoring the challenge of slums is a short-sighted and unsustainable political policy for any city or nation.