This dissertation constructs a "non-ideal theory" of justice: a systematic theory of how to respond justly to injustice. Chapter 1 argues that contemporary political philosophy lacks a non-ideal theory of justice, and defends a variation of John Rawls' famous original position–the Non-Ideal Original Position–as a method with which to construct such a theory. Finally, Chapter 1 uses the Non-Ideal Original Position to argue for a Fundamental Principle of Non-Ideal Theory: a principle that requires injustices to be dealt with in whichever way will best satisfy the preferences of all relevant individuals, provided those individuals are all rational, adequately informed, broadly moral, and accept the correct "ideal theory" of fully just conditions. Chapter 2 then argues for the Principle of Application–an epistemic principle that represents the Fundamental Principle's satisfaction conditions in terms of the aims of actual or hypothetical reformist groups. Chapters 3-5 then use these two principles to argue for substantive views regarding global/international justice. Chapter 3 argues that the two principles establish a higher-order human right for all other human rights to promoted and protected in accordance with the two principles of non-ideal theory. Chapter 4 argues that the two principles defeasibly require the international community to tolerate unjust societies, provided those societies respect the most basic rights of individuals. Finally, Chapter 5 argues that the two principles imply a duty of the international community to ameliorate severe poverty, as well as a duty to implement "fair trade" practices in international economics.