This unique volume provides a mechanistic look at key aspects of the inflammatory response seen in critical illness. Key cells and mediators involved in the innate inflammatory response and the pathways employed to combat infection or respond to injury are emphasized. It has become clear that a delicate balance exists to allow eradication of infection with minimal immune-mediated tissue injury in the process. For this reason, an up-to-date discussion of how the inflammatory response down regulates itself has been included. The inflammatory response in the critically ill is vastly different than in healthy hosts. Discussions about the mechanisms of pharmacologic immune suppression and other less commonly considered immunomodulated states seen frequently in critical care medicine have been included. Given the differences in immune function seen in critical illness, the importance of considering the immune system an organ whose function must be monitored and optimized for the best possible outcome has been highlighted. In addition, we have included up-to-date discussions of prevention and diagnostic approaches to extremely common infectious entities which must be monitored for and treated appropriately in the setting of critical illness induced immune dysfunction.