In order to quantitatively understand the conditions of blood flow through various in vitro and in vivo geometries, the flow properties of blood must be experimentally determined. In this chapter, we initially consider the rheological behavior of blood under conditions where the blood is treated as a homogenous fluid and thus where the formed elements (e.g., red blood cells, white blood cells) are tacitly ignored. This approach is then modified in order to consider flows where the blood cell characteristic dimensions approach those of the geometries in which the flow takes place. The former approach yields the macro-rheological properties of blood while the latter yields micro-rheological characteristics; in general, data obtained in geometries of 200 μm or less are considered micro-rheological. It is of interest to note that the study of blood rheology dates, at least, to the work of Poiseuille who attempted to derive an equation for blood flow in tubes. However, due to experimental difficulties associated with blood coagulation he was unsuccessful with these attempts, and thereafter turned to simpler fluids such as water and oil to develop his well-known equation [1].