Hemorheology is the science of the deformation and flow of blood and its formed elements (mainly red blood cells and to a lesser part white blood cells and platelets). This field includes investigating the bulk properties of blood, determined in viscometric experiments in macroscopic samples, and its microscopic properties. The present review will first consider the known and postulated effects of blood rheological properties on aerobic performance as related to its effects on oxygen delivery and second, the acute and long term effects of exercise on blood rheology. Blood and plasma viscosities are among main determinants of blood flow resistance both in macro- and microcirculation and of cardiac function. Red blood cell (RBC) deformability and local hematocrit have also been demonstrated to influence the oxygen diffusion capacity at the pulmonary and the muscular levels. At least, RBC aggregation properties may have either a negative or positive effect on blood flow, and thus on aerobic performance, depending on the orientation of the vessels, the shear rate in those vessels and the interaction of RBC aggregates with endothelium. During exercise, plasma viscosity and hematocrit usually increase leading to an increase of blood viscosity. Controversial results are frequently found about the influence of acute exercise on RBC deformability and aggregation, with some studies describing alterations, improvement or no changes, depending on the kind of exercise performed and on the population studied. The mechanisms underlying these changes might be attributed to white cell activation and/or lactic acid production. Also, we will discuss the effects of training, with aerobic training decreasing blood and plasma viscosities, lowering hematocrit and RBC aggregation, increasing RBC deformability.