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Offsets, and in particular international offsets, have been advanced as an important tool in climate policy, capable of significantly reducing the costs of emissions reductions. As attention turns to the existing CAA as a potential vehicle for general reduction of GHG emissions, an important question is whether regulation under the statute is compatible with international offsets. Certain regulatory programs under the CAA are likely candidates for GHG regulation, but many of them are legally incompatible with international offsets. Those programs that might permit use of international offsets have other problems that make them unpopular choices for GHG regulation. To the extent that CAA regulation depends on state action, state law and constitutional limitations appear to offer more barriers than opportunities for use of international offsets. These conclusions have implications for the costs and flexibility of climate policy under the CAA.