Of the world’s 7.4 billion population, approximately 4 billion live in urban areas. Of these, a quarter, approximately 1 billion people, live in slums! The United Nations defines a slum as a place where people live with any one of “five deprivations”: clean water, improved sanitation, sufficient living area so as not to be over-crowded, durable housing, and secure tenure. Hundreds of millions lack all five. With accelerating urban growth, in many areas slums in outlying areas, known as “informal settlements,”1 are the fastest growing. The growth of informal settlements during the latter part of the 20th century and continuing more rapidly in the 21st century raises serious concerns, both for the large number of people living without the basic requirements for a decent life as well as for the wider implications of these living situations for cities, their countries and indeed the world. Communities of people living without adequate services and spatially and psychologically divided from the cities in which they reside are a crucible for issues that extend beyond their borders. Such issues potentially include health pandemics, national and global insecurity, political instability and mass migration.