Criteria for the classification of systemic lupus erythematosus (SLE) are not sufficient to describe the degree of disease activity. Several instruments to assess disease activity have been developed. This chapter reviews the derivation, validation, and clinical application of current disease activity measures in SLE, as well as comparison among them. As patients with lupus survive longer, the sequelae of the disease activity and its therapy are becoming more common. The derivation and validation of the single, generally accepted SLICC/ACR damage index is also discussed.