From a review of the literature of blood flow it is concluded that the physical properties of blood from different animals are very similar and that its basic rheology is reasonably well understood, except for minor uncertainties regarding the effect of anticoagulants. The major difficulty lies in applying this knowledge to hemodynamics where, in vivo, there is disagreement on the nature of the flow. In general, present-day hydrodynamics is insufficiently advanced to be applied with confidence to the problem, even as a qualitative guide.