Carbon markets are substantial and they are expanding. There are many lessons from market experiences over the past eight years: there should be fewer free allowances, better management of market-sensitive information, and a recognition that trading systems require adjustments that have consequences for market participants and market confidence. Moreover, the emerging market architecture features separate emissions trading systems serving distinct jurisdictions and a variety of other types of policies exist alongside the carbon markets. This situation is in sharp contrast to the top-down, integrated global trading architecture envisioned 15 years ago by the designers of the Kyoto Protocol and raises a suite of new questions. In this new architecture, jurisdictions with emissions trading have to decide how, whether, and when to link with one another. Stakeholders and policymakers must confront how to measure the comparability of efforts among markets as well as relative to a variety of other policy approaches. International negotiators must in turn work out a global agreement that can accommodate and support increasingly bottom-up approaches to carbon markets and climate change mitigation.