Red blood cell (RBC) aggregation is a property of cells suspended in aggregating media, and is affected by both suspending phase and cellular properties (see Chapter 2). It should be noted that the suspending phase (i.e., plasma) and RBC properties may be altered under the influence of physiological and pathological processes as discussed in Chapter 8. Prominent changes in RBC aggregation can be detected in various diseases, including inflammatory states, vascular pathologies, and tissue perfusion problems inducing damage to RBC. Such alterations are reflected by various aspects of RBC aggregation. More specifically, these include: (1) the extent of aggregation (i.e., number of RBC in each aggregate), (2) the time course of aggregation (i.e., the rate of rouleaux formation), and (3) the magnitude of forces holding RBC together in aggregates.