For the past dozen years there has been a growing interest in the possibility of mitigating the global warm- ing effects of carbon dioxide by increasing the carbon stocks of biomass and soils. The earliest economics studies examined the costs of capturing and storing carbon in forest ecosystems (for a review of these stud- ies, see Richards and Stokes 2003). Over time, the potential for cost-effectively storing carbon on agricul- tural lands has also emerged as a focus of research (Lal et al. 1998, McCarl and others 2001, Antle and others 2002). Through the course of the research on carbon sequestration and policy, two themes have emerged. First, because of the nature of the analyses, it has been difficult to compare the results of the many studies that report estimates of the cost of carbon sequestration in terms of dollars per ton. Second, it appears that it will be more difficult to implement a market-based large- scale carbon sequestration program than many had imagined. These two issues have led to significant con- fusion in the discussions of carbon sequestration eco- nomics and policy. In fact, the two issues are inter- twined. It is impossible to meaningfully model the costs of a carbon sequestration program absent a careful description of how the program would be imple- mented