The protocols in this unit illustrate a range of different procedures that have been used to fractionate tissue homogenates. They emphasize different fractionation techniques that have been used for rat liver, an abundant tissue that has been a favorite of many investigators and has served as the source of many organelle preparations of excellent purity. For selected procedures, other examples have been given using other tissue sources (e.g., glandular tissues that maintain protein storage granules for regulated secretion) or, where particularly favorable, cultured cells. The basis (or goal) of each separation and the merits and limitations of each procedure are summarized to provide a guide for selecting among the various approaches. The large number of protocols include specific details relevant to a particular sample cell type (and in many cases a particular organelle).