In the following, different techniques for a heat-induced pretreatment of lignocellulosic biomass for the production of solid fuels are described. Thereby, also changes at the molecular level of the biomass are discussed. The discussed techniques (torrefaction, hydrothermal carbonization, vapothermal carbonization, and autoclaving) proceed in a temperature range up to 300 degrees C. Finally, a classification of the different techniques takes place to evaluate the specific advantages and disadvantages.