Deforestation accounts for about 25 percent of the annual global emissions of carbon dioxide. The Nature Conservancy has established a portfolio of six pilot carbon sequestration projects in Belize, Bolivia, Brazil and the U.S. that demonstrate that reforestation, forest protection, and forest management can provide scientifically valid greenhouse gas mitigation. All projects are designed to balance the need for cost-effective and measurable greenhouse gas mitigation, the protection of biodiversity, and the host country’s sustainable development objectives. If offset measurement is deemed to be unreliable by decision makers, or is too costly to investors, carbon offset projects will not be realized at a scale that is truly significant to the protection of biodiversity or sustainable development. The advancement of credible and cost-effective projects in the future is dependent upon addressing technical challenges by applying approaches on actual projects. The Nature Conservancy is conducting research to: 1) improve carbon offset estimates during both the planning and implementation phases of projects; 2) devise standardized approaches to estimating project carbon benefits at a reasonable cost; and 3) implement new projects as proving grounds. Early results suggest that while some improvements are needed, existing techniques can provide credible and cost-effective measurements.