In 1976, the OECD sponsored the "Healthfulness of Life" Common Development Effort under the direction of the Social Indicators Programme. With eight member countries and WHO participating, the group set out to develop and statistically define indicators of short and long-term disability for inclusion in population surveys. This paper concentrates on the development and validation testing of the long-term disability items. The technique adopted was one of measuring an individual's ability to perform tasks essential to daily living. The conceptual background and definitions are outlined along with the final recommandations for survey questions and indicator construction put forward to the OECD countries. In addition to the specific testing programmes carried out by the participating countries, the recommended items have been included in other large scale surveys such as the OECD Comprehensive Survey which has been carried out in several countries.