Utilizing a simple, cost effective, feasible and efficient recycling process for waste plastics, which are largely produced from non-renewable sources, is strategically important for a sustainable environment and economy. In Europe, landfilling is still the major waste management method; therefore new routes for recycling are being researched to increase the recycling rates. In this research, hydrothermal processing was used for recycling of waste carbon fibre reinforced plastics (CFRP) and printed circuit boards (PCB) in a batch reactor were investigated. Also, the applicability of the hydrothermal process was tested on refuse derived fuel (RDF), as it is a good representative of municipal solid waste which is a complex waste mixture consisting of plastics, other biodegradable materials and inorganic materials. The ability of supercritical water to degrade the resins and plastics in the composite wastes was largely influenced by the presence of different additives and/or co-solvents. Water at supercritical conditions was able to remove 92.6% of the resin from the CFRP waste in the presence of KOH and 10 wt% H2O2. In the work with PCB, 94% of the resin removal was achieved with alkalis, at zero residence time. The carbon fibre was recovered by preserving 78 % of its tensile strength due to the loss in the mechanical properties as a result of oxidation on the carbon fibre surface. When mixtures of ethylene glycol and water were used as solvent, without any addition of a catalyst, 97.6 % resin removal was achieved at 400oC. The liquid obtained from hydrothermal processing of PCB mainly composed of phenol, and phenolic compounds, which are the precursors of the original thermosetting resin. The liquid effluent from the degradation of CFRP with water and ethylene glycol mixture became too complex for recovery and so was gasified under supercritical water conditions. In the presence of NaOH and ruthenium oxide as catalysts the produced fuel gas consisted of H2, CH4, CO2, CO and C2-4 hydrocarbon gases. The carbon fibres recovered using ethylene glycol co-solvent preserved its mechanical properties and used for the manufacture of new composite materials. The mechanical tests showed that the new composites with recovered carbon fibres had enhanced mechanical properties similar to those made from virgin carbon fibres. Finally RDF was subjected to hydrothermal gasification process to produce fuel gas. Up to 93% carbon gasification efficiency was achieved in the presence of 5 wt% RuO2/γ-Al2O3 catalyst, producing a fuel gas mostly consisting of H2, CH4, and CO2 with a heating value of 22.5 MJ/Nm3. The gross calorific value of the product gas increased to 32.4 MJ/Nm3 in the presence of NaOH, as a result of carbon dioxide fixation as sodium carbonate. Also, high yields of hydrogen were obtained in the presence of both the NaOH and ruthenium catalysts, as both promoted the water-gas shift reaction.