The UN estimates that approximately 1 billion people around the world live in slums. By 2050, the UN says, there may be 3.5 billion slum dwellers, out of a total urban population of about six billion. Cities around the world are growing at an unprecedented rate and much of the growth is attributable to people moving from rural to urban areas, lured by the promise of new opportunities. Unfortunately, though, many of these new arrivals end up in living in the slum areas, steeped in dire poverty. For example, of the 500,000 people who migrate to Delhi, India each year, it is estimated that 400,000 end up in slums (Mike Davis, Planet of Slums, p. 18, 2006).
Slums are typically not found in the developed world, as evidenced by the map below. They are characterized by conditions far less habitable than "projects" or government housing -- slums are evidenced by severe overcrowding, inadequate access to safe water and sanitation and insecurity of tenure. In the sums of Kolkata (Calcutta), India, an average of 13.4 people are crammed together into a single room and, in the Dharavi slum of Mumbai (Bombay), India, an incredible 18,000 people per acre somehow dwell in 10-by-15-foot rooms stacked on top of one another.