The present thesis is about hydrothermal liquefaction of different microalgae strains and sewage sludge under varying experimental conditions. First, the current state of the art for the production of biomass derived fuels is described in detail. Then, the theoretical backgrounds of hydrothermal liquefaction of biomass are examined and optimal process conditions are determined based on data obtained from literature. The experimental part comprises preliminary tests for the optimisation and standardisation of a practicable setup and process routine, followed by three major test series. The first one includes reference tests with different materials (chlorella-, nannochloropsis and spirulina-microalgae as well as sewage sludge). Two further test series were conducted, to investigate the influence of gas atmosphere and pressure on bio-crude yields. All experiments were carried out in a high-pressure autoclave at temperatures of about 330°C and holding-times of 10 minutes. Mass balances were obtained to examine bio-crude and process gas yields as well as the amounts of filter residue in the aqueous and oil-rich phase. Selected bio-crude samples and filter residues were characterised regarding heating value and elemental composition. Process gas composition was investigated by FTIR spectrometry. Bio-crude yields (on dry matter basis) of 28,3% and 18,3% were obtained for nannochloropsis- and chlorella-microalgae respectively. Both under hydrogen atmosphere and elevated pressures a higher amount of filter residue of the dichloromethane-soluble phase was detected.