Environmental constraints and energy saving concerns are strong driving forces for improving the quality of fuels. In particular, deep hydrodesulphurization and dearomatization of diesels are mandatory. Preparation or modification of optimized hydrotreating catalysts are related to scientific bases and precise knowledge of the real active sites during operating conditions. This paper will review the updated results on the most promising model attempting to explain the catalytic synergy in hydrodesulphurization and the recent evolution of the formulation of such catalysts.