Naomi Soderstrom, managerial accounting professor at the Leeds School of Business, studies the numbers at the heart of environmental policy and compliance. Here she takes on emissions permits, the currency for the U.S. sulfur cap-and-trade system, in an attempt to define and value the powerful economic tools. Understanding how the market values permits is crucial to helping managers make cost-effective decisions about whether to sell permits, bank permits, or invest in energy improvements.