Climate change mitigation initiatives based on biological sequestration of carbon have paid little attention to biodiversity, with important implications both for climate change mitigation and for ecosystem services that depend on biodiversity. Here the chapter reviews the theoretical and empirical evidence for forest biodiversity effects on carbon sequestration. This chapter suggests that protection of primary forests is the most effective option for maximizing carbon sequestration in forest ecosystems, and should be included in future international agreements. Because carbon sequestration is a long term goal, this chapter presents the case that avoidance of losses should be emphasized over short term uptake, and that maintenance of mixtures of dominant and subdominant species and genotypes are the safest option for carbon sequestration in plantations and agroforestry systems. Biodiversity conservation should be included in the development of policy for climate change mitigation initiatives based on carbon sequestration in forested systems, including those related to the Kyoto Protocol.