We all want to be fair. And we all want to be treated fairly. But what does it mean to be fair? And what is fairness in the first place? These questions have not been studied as extensively as one might think. We review fairness theories in philosophy and economics and show what both fields can learn from each other. We also advance a number of more specific claims, such as: fairness can be understood as an objective concept; and: the principle of proportionality requires more than just applying the proportional rule.