When I applied to graduate school many years ago, I wrote an essay expressing my puzzlement at how a country that could put a man on the moon could still have people sleeping on the streets. Part of that problem is political will; we could take a lot of people off the streets tomorrow if we made it a national priority. But I have also come to realize that NASA had it easy. Rockets conform to the unchanging laws of physics. We know where the moon will be at a given time; we know precisely how fast a spacecraft will enter or exist the earth's orbit. If we get the equations right, the rocket will land where it is supposed to--always. Human beings are more complex than that. A recovering drug addict does not behave as predictably as a rocket in orbit. We don't have a formula for persuading a sixteen-year-old not to drop out of school. But we do have a powerful tool: We know that people seek to make themselves better off, however they may define that. Our best hope for improving the human condition is to understand why we act the way we do and then plan accordingly. Programs, organizations, and systems work better when they get the incentives right. It is like rowing downstream.