Using biomass to provide energy services is a strategically important option for increasing the global uptake of renewable energy. Yet the practicalities of accelerating deployment are mired in controversy over the potential resource conflicts that might occur, particularly over land, water and biodiversity conservation. This calls into question whether policies to promote bioenergy are justified. Here we examine the assumptions on which global bioenergy resource estimates are predicated. We find that there is a disjunct between the evidence that global bioenergy studies can provide and policymakers' desire for estimates that can straightforwardly guide policy targets. We highlight the need for bottom-up assessments informed by empirical studies, experimentation and cross-disciplinary learning to better inform the policy debate.