Benefit-sharing mechanisms are a central design aspect of REDD+ because they help to create the necessary incentives to reduce carbon emissions. However, if stakeholders do not perceive the benefit sharing as fair, the legitimacy of REDD+, and support for the mechanism, will be weakened. In this paper, drawing on data from CIFOR's Global Comparative Study on REDD+, we analyze national policy processes in 6 countries and incipient benefit-sharing arrangements in 21 REDD+ project sites. Through our analysis of current practices and debates, we identify six rationales that have been put forward to justify how benefits should be distributed and to whom. These rationales encompass a range of perspectives. Some hold that benefit sharing should be related to actual carbon emission reductions or to costs incurred in achieving the reduction of emissions; others emphasize the importance of a legal right to benefit, the need to consider aspects such as poverty reduction or the appropriateness of rewarding those with a history of protecting the forest. Each rationale has implications for the design of benefit-sharing mechanisms and the equity of their outcomes. We point out that, given the wide range of rationales and interests at play, the objectives of REDD+ and benefit sharing must be clearly established and the term "benefit" defined before effective benefit-sharing mechanisms can be designed. For stakeholders to support REDD+, the legitimacy of decision-making institutions, consideration of context, and attention to process are critical. Building legitimacy requires attention not only to fair distributional outcomes but also to consensus on relevant institutions' authority to make decisions and to procedural equity.