Human red blood cells (RBC), as well as RBC from most mammals, have a tendency to form aggregates that initially consist of face-to-face linear structures that resemble a stack or roll of coins (Chien and Jan 1973; Chien and Sung 1987; Fåhraeus 1929). Such individual linear structures are often termed rouleau, with rouleaux being the plural. Figure 1.1 shows a representative assortment of rouleaux for normal human RBC in autologous plasma, where it is obvious that (1) the number of RBC per rouleau can vary widely, and (2) side-to-side or side-to-end branching can occur. While Figure 1.1 represents normal aggregation in a thin two-dimensional geometry (i.e., between a microscope slide and cover slip), rouleaux can form threedimensional structures under appropriate conditions (Branemark 1971; Cokelet and Goldsmith 1991). As anticipated, the formation of these larger structures is affected by factors such as available space and the level of cell-cell attractive forces.