The clinical entity, definitions, and the significance of SIRS (systemic inflammatory response syndrome) were reviewed. The term, SIRS was proposed to define sepsis and its sequelae clearly in 1991, in order to make early detection of the disease possible, and to improve the ability to compare innovative potential diagnostic and therapeutic modalities by standardizing terms. Although the criteria of SIRS is not strict and too sensitive, SIRS has been shown to be useful as a warning sign of severe condition in clinical setting. We also discussed about a new concept, CARS (compensatory anti-inflammatory response syndrome), which was characterized as anti-inflammatory mediators-dominant condition, in this issue.