It is widely agreed that oppression is wrong. But what, above all, makes it wrong? What is its chief wrong-maker? This paper argues that what chiefly makes oppression wrong is that it violates two principles of political morality. The first is the principle of wrongful benefit: no one should benefit from her own wrong. The second is the principle of institutionalized harm: no social group should be subjected to an unjustified institutionalized harm. Let us call this explanation of oppression's wrongness the two principles thesis. In arguing for this thesis, I give an account of the nature of oppression, explicate the two principles, prove that they both exist, and then show how oppression violates them and thus is wrong. I then defend the thesis on the grounds that it better explains oppression's wrongness than do two rival explanations. First, the view that oppression violates a principle of non-domination. Second, the view that it violates a principle of equal respect. I show that my explanation better satisfies the nine criteria for choice among explanatory theories: evidential adequacy, simplicity, unifying power, falsifiability, testability, neatness, conservativeness, fecundity, and mechanism-informativeness.