Whilst the life expectancy of products in non-demanding applications have traditionally been predicted from previous in-service experience (i.e. service conditions considered identical to those of which data already exists), long-term or critical applications require the use of accelerated ageing regimes to generate data commensurate with the engineering requirements and life expectancy of the component. This Good Practice Guide is intended to give guidance on accelerated ageing and performance testing of polymeric materials for generating design data and for quality assurance purposes. The document is primarily concerned with polymers (thermosets and thermoplastics) and fibre-reinforced plastics, and considers a range of hostile environments including artificial weathering, hot/wet and chemical (inc. solvents, caustic and acidic solutions) exposure.