The SLICC/ACR damage index for SLE was developed to assess accumulated damage since the onset of the disease. The damage includes non-reversible changes in organs and systems affected by the disease process itself, its therapy, or inter-current illness. This paper describes the development of the damage index, its validation and its use. It is recommended as an outcome measure for longitudinal studies of prognosis and response to new therapies, and as a stratification measure for clinical trials.