The mechanisms of red cell aggregation and cell deformation can impart viscoelastic behavior to blood: at very high hematocrits, the cell deformation mechanism dominates; at physiological and low hematocrits, red cell aggregation dominates at low shear rates. At physiological hematocrits, the viscoelastic behavior may be linear at low shear rates, where the elastic component of the complex viscosity may be comparable in magnitude to the viscous component; in the higher shear rate region, where red cell aggregation is less extensive or absent, blood behavior is nonlinear, and the elastic component becomes less significant. The nonadditivity of steady and oscillatory flow data for prediction of pulsatile flow behavior is indicative of the importance of the mean shear rate and the kinetics of the red cell aggregation-disaggregation processes in governing pulsatile blood flow. While oscillatory measurements will be useful in assessing rheological parameters that may give insight into the fundamental aspects of flow of normal and pathological bloods, it is not clear that the elastic component of the complex viscosity of blood will be of significance in physiological pulsatile flow. Many interesting questions remain to be answered, such as the question raised by the finding that normal stress differences were not detectablemore » for blood under low shear rates. Judging from the number of papers presented at the Third International Congress of Biorheology, we can look forward to considerable activity in this area.« less