In the open literature, there are numerous papers related to the chemical and physical properties of unsupported transition metal sulfides. Besides characterization studies, the performance of a large variety of mono-, bi- and multi-metallic materials in various test reactions has been examined. The performance studies identified several unsupported materials with higher activity and/or selectivity than the traditional gamma-Al2O3 supported Ni/Co-Mo/W catalysts. This has resulted in numerous patents disclosing the preparation of unsupported transition metal sulfides and their use as hydroprocessing catalysts. However, for several reasons (high price and too high activity for existing refinery process equipment), the commercial use of unsupported catalysts in refinery processes has so far been limited to the NEBULA (R) catalysts. Catalysts based on NEBULA technology provide unprecedented activity for hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and aromatics saturation (HDA). (C) 2007 Elsevier B.V. All rights reserved.