Reserves of fossil primary energy carriers are limited. Consequently liquid secondary energy carriers especially for mobile applications made from fossil reserves will not carry on forever but need to be replaced in a not-to-far future. Two substitution strategies are currently under investigation – the use of oil from plant seeds either directly or after chemical modification (biodiesel) or the gasification of complete plants, use of the product gases (mainly CO and H2) in a Fischer-Tropsch process with subsequent refining. A third possible pathway would be the so-called direct liquefaction, i.e., the conversion of complete plants into liquid fuels without gasification. This process is discussed and various technical implementations are critically evaluated in the present paper.