We present a review of blood rheology with special emphasis on its application in anesthesiology. The rheological behavior of blood is determined by 2 variables, non-Newtonian viscosity and yield stress. The physical significance of these quantities is discussed. Blood viscosity directly affects total peripheral resistance, and changes in the state of peripheral vessels cannot be accurately evaluated unless simultaneous measurements of blood viscosity are made. Blood viscosity also influences cardiac output, and elevations in hematocrit may reduce total O2 transport by increasing viscosity to the point that cardiac output decreases. The role of blood viscosity and blood yield stress in the pathogenesis of deep-vein thrombosis is mentioned, and the role of anesthesia in affecting viscosity by decreasing venous flow is discussed. Clinical examples of the role of blood rheology in neonatal respiratory distress and during open heart surgery are also given.