Sepsis is among the most common reasons for admission to intensive care units throughout the world. In 1991, a new set of terms and definitions was developed to define sepsis more precisely. The concept of the 'systemic inflammatory response syndrome' (SIRS) was developed, and its diagnostic criteria were defined. Sepsis was defined as suspected or microbiologically proven infection together with SIRS, while severe sepsis was defined as sepsis together with sepsis-induced organ dysfunction. Septic shock was defined as sepsis-induced hypotension persisting despite adequate fluid resuscitation. Data from recently published trials support this hierarchical stratification, with the mortality from sepsis ranging from 10% to 15%, severe sepsis from 17% to 20%, and septic shock from 43% to 54%. The distinction between severe sepsis and septic shock is critically important as it stratifies patients into groups with a low and a high risk of dying, respectively. However, currently the diagnostic criteria of septic shock remain vague. We suggest that septic shock is best defined by a systolic blood pressure less than 90 mmHg (or a fall in systolic blood pressure of > 40 mmHg), or a mean arterial pressure less than 65 mmHg after a crystalloid fluid challenge of 30 mL per kg body weight in a patient with severe sepsis. We believe that a vasopressor should be initiated in patients who remain hypotensive after this fluid challenge. The above operational definition of septic shock is important, as it clearly and unambiguously defines in which patients, and when, treatment with a vasopressor should be initiated, and in which patients adjunctive therapy with hydrocortisone and drotrecogin alfa (activated) should be considered.