The concept of ‘carbon debt’ and carbon payback time with reference to bioenergy and biofuels was probably launched by anarticle in Science in 2008. The concept is increasingly seen as an indicator of the sustainability of bioenergy supply chains.Particularly for forest bioenergy supply chains the time lapse between harvest and regrowth may be a signifi cant factor for themodeled carbon debt. A meta-analysis of more than 250 model scenarios was conducted to evaluate the factors and assumptionsdetermining carbon debts and payback time of forest bioenergy supply chains. Factors such as spatial and temporal scale, biome,origin of the wood resource, which fossil fuels are displaced, forest history, baseline scenario, accounting principle, and databackground were included in the analysis. This paper discusses the evolution of the carbon debt concept, how different factorsand assumptions infl uence the outcome of carbon debt studies, the reproducibility of carbon debt analyses, and the applicabilityof the concept as a measure of sustainability of forest bioenergy supply chains