Deterioration of the quality of oilstock makes it an urgent problem to use non-traditional hydrocarbon materials (heavy oil, bitumen, residues etc.). Processing of heavy oilstock (HOS) needs new methods that would be effective for the yields of light fractions, suppression of coke formation, and saturation of the liquid products with hydrogen. At the same time, expenses of HOS extraction and transportation are several times as high as those of light and middle oils that makes it necessary to minimize the capital and operational costs for the process. The review paper integrates for the first time the results of the studies of catalytic steam reforming of HOS as a potential alternative of the traditional processes for HOS upgrading based on decreasing the carbon content (thermal cracking, visbreaking, catalytic cracking) or on the saturation of liquid products with hydrogen (hydrocracking). Under discussion are main distinctions of the process from HOS upgrading with water (hydropyrolysis in sub- or supercritical water) as well as specific features of the catalytic steam cracking as dependent on the process parameters and catalysttype and putative mechanisms of the participation of water in the process.