Recently, there has been an explosion of life-cycle assessment (LCA) studies of biofuels to support biofuel policy making. It is difficult to draw general conclusions from the set of studies due to the variation in outcomes. Causes of this variation include real-world differences, data uncertainties and methodological choices. In this review we explore some of the more complicated sources of differences in findings related to LCA methodology by reviewing 67 LCA studies published between 2005 and 2010. A very important and particularly difficult problem to solve is coproduct allocation. Different allocation methods, all approved under the International Organization for Standardization (ISO) standard for LCA studies, can cause improvement percentages compared with fossil fuels to vary from negative to above 100%. The treatment of biogenic carbon is another important issue. Most studies include neither extractions nor emissions of biogenic CO2, but a number of these LCAs do include both, leading to very different conclusions on greenhouse gas performance of biofuel chains.