The mammalian erythrocyte is a very suitable model for the study of aging at the cellular and molecular level. It is not only a matter of apparent simplicity in terms of biochemistry, biophysics and physiology but more likely this cell offers a great possibility for elucidating some basic problems in the process of aging. In fact, nowadays, it is possible to follow individual cells all along their life span in circulation, it is possible to obtain these cells when young, middle aged or old and it is possible to obtain cells from individuals of defined ages and transfuse them into compatible recipients to investigate the role of the environment where the cell lives, and finally it is possible to easily manipulate the red cell content in terms of enzymatic activities and/or metabolic properties to investigate the possible effect of these manipulations on cell survival. This book, Red Blood Cell Aging, is based on a symposium held in Urbino, Italy, at the end of 1990 and examines the impact of age on the membrane, metabolism, structural and enzymatic proteins of mammalian erythrocytes. The various contributions to this symposium not only described those processes of aging which affect the cell but also provided a nearly complete picture of the event{s} and mechanism{s} that every day permits to recognize among 25 trillion circulating red cells {in an average adult} that 1 percent that have reached the end of their 120 day life span in circulation.