Most of these 24 papers are from the Second Symposium on Advances in Hydrotreating Catalysts, August 1994; the remainder were solicited from individuals covering areas not included in the symposium. The field must keep pace because of the evermore stringent requirements for removal of sulfur and nitrogen that (we hope) will be demanded by refiners and regulators throughout the world. Topics include the biological processing of hydrocarbon feeds, reaction mechanism and pathways, structural models of hydrotreating catalysts and their characterization, and new catalytic materials.