Maintenance and restoration of intravascular volume are essential tasks of critical care management to achieve sufficient organ function and to avoid multiple organ failure in critically ill patients. Inadequate intravascular volume followed by impaired renal perfusion is the predominate cause of acute renal failure. Crystalloid solutions are the first choice to correct fluid and electrolyte deficits in these patients. However, in case of major hypovolemia, particularly in situations of increased capillary permeability, colloid solutions are indicated to achieve sufficient tissue perfusion. Whereas albumin should be avoided for correction of intravascular hypovolemia, synthetic colloids can restore intravascular volume and stabilize hemodynamic conditions. In addition to a faster, more effective and prolonged restoration of intravascular volume, colloid solutions are able to improve microcirculation. Of the synthetic colloids, hydroxyethyl starch (HES) solutions with a low in vivo molecular weight, such as HES 200/0.5, offer the best risk/benefit ratio. These solutions are safe with respect to effects on coagulation, platelets, reticuloendothelial system, and renal function, if used below their upper dosage limits. For patients with acute renal dysfunction, daily monitoring of renal function is necessary if colloids are required to stabilize hemodynamic conditions. In these patients, measurement of the colloidal osmotic pressure and adequate amounts of crystalloid solutions will reduce the risk of hyperoncotic renal failure. Of all colloids, gelatin and HES solutions with low in vivo molecular weight are preferred in these cases. In the very specific situation of kidney transplantation, colloid solutions should be administered in a restricted manner to organ donors and kidney recipients.