The use of hydrogen as clean fuel gas in the power generation sector becomes essential to reduce the environmental issues related to conventional fuel usage. By avoiding biomass drying process, supercritical water gasification is considered the most efficient technology in hydrogen production from wastewater sludge. Wastewater sludge is difficult to disposal in its received form since it is often produced with high moisture content, contribute to numerous environmental issues and direct contact with this waste can result in health concerns. The assessment of the treatment and conversion of this material into fuel gas at condition beyond supercritical state (374 degrees C and 22.1 MPa) is required. This paper is discussed the degradation routes of wastewater sludge in supercritical water. Furthermore, it is reviewed the influence of the main operation parameters role in the hydrogen production, which includes reaction temperature, pressure, residence time, feed concentration and catalysts. The development in reactor design and setup for maximum hydrogen production is highlighted. The technical challenges encountered during the conversion process and its solutions are also discussed. In addition, future prospective to optimal and standardization of the supercritical water gasification process is reviewed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.