The understanding and measurement of blood viscosity play an important part in mordern medicine. The basic principles and relevances of fluid viscosity are discussed. Factors which alter viscosity include shear rate, haematocrit, erythrocyte deformability, internal fluidity, pH, plasma proteins, temperature, osmolality, lipids and leucocytes. In vivo changes in vascular architecture are thought to influence blood viscosity. Some of the clinical conditions in which viscosity is important are described. Hyperviscosity should be treated before complications arise.