Emissions Trading, under the Clean Air Act, includes bubbles, netting, emission offsets, emission reduction banking, and is the subject of an EPA proposed Policy Statement (47 Fed. Reg. 15076, Apr. 7, 1982). These alternatives involve the creation of surplus reductions at certain stacks or vents and use of these reductions to meet requirements applicable to other emission sources. Emissions trades can provide more flexibility, and may therefore be used to reduce control costs, encourage faster compliance and free scarce capital for industrial revitalization. Where private parties have banked surplus emission reductions for future use or sale, states may determine that a portion of these reductions is required for clean air purposes. This paper provides legal analysis of the Constitutional issue of whether by 'taking' all or part of a person's banked emission reduction credits a state becomes liable to compensate that person. The paper concludes that the state would not be liable in this circumstance, particularly if the state had set out in advance, in its banking rule, the circumstances that could give rise to such a taking and the way it would be done.