Sepsis is a systemic, deleterious host response to infection leading to severe sepsis (acute organ dysfunction secondary to documented or suspected infection) and septic shock (severe sepsis plus hypotension not reversed with fluid resuscitation). Severe sepsis and septic shock are major healthcare problems affecting millions of individuals around the world each year, killing one in four and often more and increasing in incidence. The septic response is an extremely complex chain of events involving inflammatory and anti-inflammatory processes, humoral and cellular reactions and circulatory abnormalities. The inflammatory response can lead to damage to host tissue and the prolonged anti-inflammatory responses can lead to leukocyte reprogramming and dysfunctional changes in immune status. The diagnosis of sepsis and evaluation of its severity is complicated by the highly variable and non-specific nature of the signs and symptoms of sepsis.