Energy recycling is an important issue that should be developed urgently to remarkably reduce wastes and to utilize them as alternative renewable energy resources. However, a significant social problem takes place due to emission of toxic components in exhaust gas and ash. Thus the development of environmentally harmonious systems is required for practical application. A number of R & D on effective energy recycling technologies of wastes including combustion and gasification have already been carried out. Some new commercial or pilot plants to replace conventional furnaces have been constructed, tested and put into operation. This paper overviews the recent technologies in energy recycling processes of wastes, and also summarizes and reviews the main studies performed by the present authors and other researchers from Japan and abroad on the fundamental knowledge necessary for the plant design of combustion and gasification furnaces. Studies on the reaction kinetics of combustion and gasification rates of various RDFs, the emission of dioxins and hydrogen chloride, their interaction and processes to reduce emissions, and treatments for de-SOx, de-NOx and ash or slug are reviewed. Additionally based on these works, the future prospects for energy recycling processes are considered.