The conversion of biomass to fuels is advancing on two fronts: first, to enhance the usability of biomass and second, to generate the best fuel molecule(s). Lignin generally presents a barrier to biomass utilization; this problem may be circumvented by the genetic alteration of lignin-producing plants. Plant cellulosic material will need to be utilized more efficiently, requiring a greater knowledge of the multiprotein complex, the cellulosome. Many biofuel types are being considered: alcohols, esters, ethers, and hydrocarbons. Intrinsic alcohol toxicity to cells may impose limitations on ethanol and n-butanol production. Biodiesel, or fatty acid esters, can now be synthesized via recombinant Escherichia coli and may offer an improvement over alcohol fermentations. Biopetroleum (hydrocarbons) is being considered as an alternative biofuel.