This paper describes essential aspects of the hydrotreatment of pyrolytic oils in the light of results obtained until now at the Universite Catholique de Louvain. Stability of pyrolysis oils necessitates a two-step processing. A low temperature hydrotreatment enables stabilization through reactions like olefin, carbonyl and carboxylic groups reduction. Further hydrotreatment aims at hydrodeoxygenation of phenols and hydrocracking of larger molecules. Results about catalysts, reaction conditions and parameters enabling or influencing the control of the reaction are summarized. Based on these laboratory data, a preliminary techno-economical evaluation is made. 50 wt.-% yields in hydrocarbons for deep hydrorefining of pyrolysis oils can be expected. Nevertheless, a moderate hydroconversion with partial elimination of oxygen would be, economically, more advantageous.