Picture a list of the strongest clues to how long you will live and how sick you might become. Income, years of schooling, and job status would be near the top—often beating whether you have a great doctor or live next to a gym. This link is so strong that it shows up in every country, in every era, and at every rung of the social ladder. In this chapter we explore why a person’s socioeconomic position is so tightly connected to their health, and we unpack six big ideas that try to explain the pattern.