This dissertation investigates the optimal design of international climate policy. Using Integrated Assessment Models (IAM) where countries differ in income, climate impacts, exposure to energy markets, and international trade, I analyze the optimal design of carbon taxation, climate agreements, and the spillovers of different climate policies, with an emphasis on redistribution motives. I first characterize the optimal taxation of carbon depending on the availability of international transfers. I show that when redistribution is limited, the uniform carbon tax differ from the Pigouvian First Best due to these redistribution considerations and general equilibrium effects on the energy markets. Moreover, optimal carbon pricing should vary across countries, with poorer and hotter countries facing lower carbon taxes than richer, colder ones. Second, I quantify the redistributive effects of unilateral and multilateral climate policies, highlighting the role of international trade in magnifying both the costs and spillovers of climate action. Through a sufficient-statistics approach, I decompose welfare effects across these different channels and map them into observable shares and measurable elasticities. I show that leakage effects from unilateral climate action are an order of magnitude larger than the direct benefits from emission reductions, emphasizing the necessity of global cooperation. Finally, I study the optimal formation of climate agreements, in the spirit of Nordhaus' "climate clubs", navigating a trade-off between carbon reduction at the intensive margin, and an extensive margin when countries can strategically free-ride and participation is endogenous. I find that the optimal climate club includes all countries except major fossil fuel producers such as Russia, Saudi Arabia, Nigeria, and Iran, implements a $110 carbon tax within the club, and imposes a 50% tariff on non-members to deter free-riding. I also explore the role of climate finance transfers, carbon tariffs, and trade retaliation on the feasibility and stability of climate agreements. Together, these results offer new insights into how climate policies can be optimally designed and coordinated across diverse economies to address the global challenge of climate change.