The biggest challenge is that cities are growing, but in an unplanned way and in a way that creates unsafe conditions for many. The growth is steady but steadily built on steady winds up being exponential. People come for work. They know somebody in the city—a relative or friend. Often, the opportunities they find are informal. In many cases, the cities aren’t ready for them. The planning systems are out of date. The building codes don’t work. Basic services aren’t there. The conditions can be awful—overcrowded and polluted. The indicators of a slum include inadequate access to safe water or sanitation, overcrowding, poor structural quality of dwellings—whether because of the materials or the location—and insecurity of tenure. These are baselines to improve on. Adequate access to water does not have to mean running water in the house; it might mean you can pump drinkable water nearby. Similarly, access to sanitation doesn’t mean that you have a flush toilet. The ability to turn on a tap and get a glass of clean water, and the ability to close the door to a bathroom that’s in your own home are luxuries in a lot of places.