Increased intra-abdominal pressure (IAP) may occur in a number of different situations encountered by intensivists, such as tense ascites, abdominal hemorrhage, use of military antishock trousers, abdominal obstruction, during laparoscopy, large abdominal tumors and peritoneal dialysis.1-3 Both clinical and experimental evidence indicate that increased IAP may adversely affect cardiac, renal, respiratory and metabolic functions.1-5 Despite this, increased IAP is rarely recognized and treated in Intensive Care Unit (ICU) settings. There appears to be two reasons for this: the physiologic consequences of increased IAP are not well know, to most physicians and, more importantly, the capability of easily measuring IAP has not been well documented. In this chapter, we will discuss: 1) the different methods proposed to evaluate IAP in ICU; 2) the physiopathological consequences of increased IAP; 3) the existing clinical data about IAP in critically ill patients. Considering overall our data, we can conclude that: 1) different techniques are available at the bedside to estimate the IAP; 2) the IAP ranges between 10 and 20 cmH2O, substantially increased compared to normal subjects. Most of the patients have IAH, while few of them (<5%) present clinical characteristics of ACS; 3) the IAP is different among different categories of patients and its increase is not limited to surgical patients only; 4) the increase in IAP appears to influence respiratory function, homodynamic, kidney, gut and brain physiology; 5) the IAP seems to be correlated with severity scores but its relation to mortality is controversial; 6) the routine measurements of IAP by means of bladder pressure are not associated with an increased rate of urinary tract infections.