Aquathermolysis process is one of the key and economical viscosity reduction technologies being considered for the successful exploitation of huge heavy oil deposits across the world. The paper substantially reviewed the progress made and the fundamental issues related to catalysts development. The most appropriate catalysts are those containing strong, active sites that can actively break the C-C, C-S, C-O and related bonds in resins and asphaltenes, with a net increase in the concentration of saturates and lighter aromatic hydrocarbons. Solid catalysts based on natural zeolites, heteropoly acids and modified zirconia oxides have so far demonstrated excellent activities. Catalysts such as alloys of Mo, W, C and Ni, nano oxides of Cu and Fe, Ni-chelates, supported nano-Fe particles and Cu2+ and Fe3+ complexes of toluene sulfonic acid were also recently reported as good candidates for the aquathermolysis reactions. Hydrophobic zeolites prepared in the fluoride or organic silane routes, carbides of W and Mo are prospective catalysts for the future due to their excellent performance in related reactions. In all cases, further studies are required to explore the optimal catalyst synthesis procedures, operation conditions and actual reaction mechanisms. (c) 2015 Elsevier Ltd. All rights reserved.