Red blood cell (RBC) aggregation is a physiological phenomenon with important hemodynamic consequences (see Chapter 7). The degree of RBC aggregation is determined by both cellular and suspending phase properties (see Chapter 2) and changes in these properties result in various degrees of aggregation in RBC suspensions (e.g., blood). Alterations of RBC aggregation due to physiological and pathophysiological challenges have been reported extensively; this chapter is devoted to a discussion of such variations in RBC aggregation. Alterations of RBC aggregation during the course of a variety of disease or clinical states are also summarized briefly in Section 8.3.