Oxygen supply to all tissues is possible only in a condition of adequate blood circulation. Oxygen demand is the driving force that is responsive of hemodynamic adjustment. The human body acts on four modulators (intravascular volume, inotropy, vasoactivity, chrono-tropy) in order to adjust the hemodynamic state. Hemodynamic monitoring consists of techniques able to assess the hemodynamic status and to understand the mechanism of patient decompensation; its goal is to maintain adequate tissue perfusion through appropriate therapeutic interventions. An early diagnosis of hemodynamic alteration is crucial for an early treatment; several reports have explored the effectiveness of hemodynamic manipulations and results are conflicting: too many variables can, in fact, modify the results: timing and lenght of the treatment, drugs used, etc. However, at least, in some specific settings, as sepsis, early intervention has a positive impact on mortality. In this presentation it will be briefly analyzed the most common parameters used in the ICU. Arterial pressure, central venous pressure, pulmonary artery catheter derived parameters, SvO2 and their relation with organ perfusion are considered and positive and negative aspects of this type of monitoring is reviewed. Starting from these considerations we would like to underline the importance of understanding the physiological basis of monitoring and the correct interpretation of data in order to have improvement on patient outcome.