The microcirculation is a complex system, which regulates the balance between oxygen demand and supply of parenchymal cells. In addition, the peripheral microcirculation has an important role in regulating the hemodynamics of the human body because it warrants arterial blood pressure as well as venous return to the heart. Novel techniques have made it possible that the microcirculation can be observed directly at the bedside in patients. Currently, research using these new techniques is focusing at the central role of the microcirculation in critical diseases. Experimental studies have demonstrated differences in microvascular alterations between models of septic and hypovolemic shock. In human studies, the microcirculation has most extensively been investigated in septic syndromes and has revealed highly heterogeneous alterations with clear evidence of arteriolar-venular shunting. Until now, the microcirculation in acute heart failure syndromes such as cardiogenic shock has scarcely been investigated. This review concerns the physiologic properties of the microcirculation as well as its role in pathophysiologic states such as sepsis, hypovolemic shock, and acute heart failure.