This book introduces conversion of two Estonian oil shales and two biomass species and their blend to liquid oil in the medium of supercritical water. Elaboration of methodology for separation of the conversion products by extraction with different solvents. Determination of composition of liquid and gaseous products. Comparision of the yields of the products obtained from two different oil shales, two different biomass species, and oil shales and biomass species. Investigation of synergistic effect of co-liquefaction of fossil and renewable feedstock.