Mammalian blood is a rheologically complex fluid composed of cells (i.e., red blood cells [RBC], white blood cells, platelets) with differing mechanical properties suspended in a Newtonian aqueous solution of micro ions (e.g., calcium, potassium, sodium), and protein macromolecules such as albumin and fibrinogen (Rampling 2007). It is a crowded suspension, usually with a volume fraction of RBC, termed hematocrit, of about 0.40-0.45 l/l (i.e., 40-45%), and hence cell-cell interactions are the rule rather than the exception (Goldsmith 1968; Goldsmith et al. 1983). These cell-cell interactions are due to both shear forces and cell collisions as well as to attractive forces between cells (i.e., RBC aggregation). As a result, in vitro measurement of blood rheology can be difficult and requires attention to several potential artifacts. The following material describes systems for viscosity measurement, areas where problems may occur, and the rheology of blood with special emphasis on the effects of RBC aggregation.