An Advanced Thermal Treatment (ATT) process that thermally decomposes waste in the absence of oxygen at temperatures typically above 430°C. Pyrolysis produces combustible gases, liquids (bio-oil), and solid char that can be used for energy recovery or as chemical feedstock. Unlike incineration, pyrolysis prevents complete combustion, offering potential for higher-value outputs. Development in the UK has been slower than anticipated for mixed residual waste.