Discussions of ethics and climate change often focus on issues of justice. With respect to the ethics of climate change institutions, however, justice is the wrong lens. Since institutions in world politics are shaped by interests and power, they uniformly fail to meet any universal standards of justice. Differences between various theories of justice are immaterial for policy decisions, since actual institutional procedures and outputs fall short of the standards that any coherent theories would prescribe. For practical policy analysis, it is more important to focus on legitimacy than justice. For an institution to be legitimate means that it is worthy of our obedience within its sphere of activity. Legitimacy is a lower standard than justice, but still provides a meaningful ethical benchmark, and adequate legitimacy should be a necessary condition to support multilateral institutions. A cap-and-trade architecture, with compliance arrangements involving buyer liability, provides the best way of building climate institutions that are both legitimate and effective.