The aviation sector must confront rapidly increasing greenhouse gas (GHG) emissions and ambitious decarbonization targets. Alternative jet fuels (AJFs) could be route to decarbonizing this sector, though these fuels can vary widely in feedstocks used, cost, and environmental performance. To facilitate the transition to AJF, Boeing has begun testing the technical suitability of HEFA+ (Hydroprocessed Esters and Fatty Acids+) on flights as a petroleum jet fuel substitute. HEFA+ proponents have touted this fuel as a promising AJF due to its low cost—producing HEFA+ is similar to manufacturing renewable diesel in the road sector, a technology that has already reached a commercial scale and is cheaper than some other AJF pathways. This working paper evaluates the state of HEFA+ deployment and its potential to reduce the climate impacts of aviation. This study assesses the production of HEFA+ and how it differs from other AJFs, as well as the impact of feedstock choice on the sustainability and GHG performance of the finished fuel. We also assess the state of deployment, near-term availability and policy implications of expanding HEFA+ deployment within the aviation sector and the risks of competition for feedstocks with the road sector.