Sepsis, severe sepsis, and septic shock represent increasingly severe systemic inflammatory responses to infection. Sepsis is common in the aging population, and it disproportionately affects patients with cancer and underlying immunosuppression. In its most severe form, sepsis causes multiple organ dysfunction that can produce a state of chronic critical illness characterized by severe immune dysfunction and catabolism. Much has been learnt about the pathogenesis of sepsis at the molecular, cell, and intact organ level. Despite uncertainties in hemodynamic management and several treatments that have failed in clinical trials, investigational therapies increasingly target sepsis induced organ and immune dysfunction. Outcomes in sepsis have greatly improved overall, probably because of an enhanced focus on early diagnosis and fluid resuscitation, the rapid delivery of effective antibiotics, and other improvements in supportive care for critically ill patients. These improvements include lung protective ventilation, more judicious use of blood products, and strategies to reduce nosocomial infections.