A major long-term objective in the evolution of advanced fibre composites as aircraft structural materials must be the establishment of a damage tolerance methodology. The availability of this methodology would greatly aid the design of safe efficient composite structures as well as their management in service. There are, however, many difficulties in meeting this objective, including the multiplicity of failure modes in the composites, the numerous types of potentially significant defects, which may arise during manufacture or in service, and sensitivity to moisture and temperature. This paper briefly considers the present status of the damage tolerance approach for composites and then highlights the critical issues as they relate to the problem of delamination damage, an aspect of primary concern. Selected theoretical and experimental studies in the literature, providing at least partial answers to some of these issues, are briefly discussed and a unified theoretical approach is proposed.