Accurate modelling of the absorption of moisture into engineering polymers is needed to design exposure procedures for durability assessment and carry out reliable lifetime predictions. This needs to be supported by good quality data obtained under representative conditions. This Guide describes methods for measuring the absorption and diffusion of moisture in polymeric materials, including plastics, adhesives and composites. Factors that will affect moisture absorption, including service conditions (temperature, pressure and stress) and the manufacture process (e.g. state of cure), are also covered.