A study has been made of the effect of prior damage on the tensile strength and toughness of a number of carbon and glass fibre reinforced plastics materials. This prior damage was introduced by tension fatigue cycling, by transverse compression, by repeated impact, and by stress corrosion. The effects of the damage were assessed by measuring the un-notched and notched tensile strengths. Acoustic emission monitoring and microstructural studies were carried out in support of the mechanical tests. Microstructural damage can result in independent changes in the notched and un-notched strengths of a composite, although the pattern of changes is not simple.