Hydrogenation and hydrogenolysis of molecules derived from plant biomass are reactions often performed in liquid phase because the highly functionalized biomass molecules (e.g. cellulose, hemicellulose, sugars, and lignin) cannot be vaporized without undergoing thermal decomposition. The need for a solvent for biomass conversion adds another important variable to the design of catalytic systems. In this chapter, solvents and solvent effects on hydrogenation and hydrogenolysis of biomass-derived molecules are addressed in detail. In the first section, key aspects of the physical chemistry of plant biomass and hydrogen solubilization are presented. In the second section, solvent properties commonly used to rationalize solvent effects on heterogeneous catalysis are discussed. In the third section, the solvent role in competitive adsorption, activity, and selectivity is addressed for selected case studies.