There’s a market growing in the United States, but unlike markets that trade in tangible commodities, this one trades in the absence of something no one wants: greenhouse gases in the atmosphere. Hundreds of companies make it possible for individuals, organizations, businesses, and even events such as rock music festivals to proclaim themselves carbon-neutral by paying someone else to reduce their emissions. Worried about your carbon footprint? No problem. For fees of US$2–50 per ton of “avoided emissions,” an offset provider will funnel your money into an activity or technology that keeps greenhouse gases out of the atmosphere. The question is, are offset buyers really getting what they paid for?