The performance of hydroprocessing units can be influenced by the selection of the catalysts and the type of reactor to suit a particular feed. Catalysts ranging widely in chemical composition and physical properties are available commercially. New catalysts have been developed for deep hydrodesulfurization and aromatics hydrogenation. Several types of reactors are proven commercially as well. The catalysts and reactors selected for light feeds differ markedly from those selected for heavy feeds. Fixed-bed reactors have been traditionally used for light feeds. High asphaltene and high metal content feeds are successfully processed using moving-bed and/or ebullated bed reactors. Multi-reactor systems consisting of moving-bed and/ or ebullated bed reactors in series with fixed-bed reactors can be used to process difficult feeds. For heavy feeds, the physical properties (e.g., porosity), shape and size of the catalyst particles become crucial parameters. Pretreatment of catalysts by presulfiding improves the performance of the units.