Presently, the need for methods involving separation, identification, and characterization of different kinds of cells is amply realized among immu- nologists, hematologists, cell biologists, clinical pathologists, and cancer researchers. Unless cells exhibiting different functions and stages of differ- entiation are separated from one another, it will be exceedingly difficult to study some of the molecular mechanisms involved in cell recognition, specialization, interactions, cytotoxicity, and transformation. Clinical diag- nosis of diseased states and use of isolated cells for therapeutic (e. g. , immunotherapy) or survival (e. g. , transfusion) purposes are some of the pressing areas where immediate practical benefits can be obtained by applying cell separation techniques. However. the development of such useful methods is still in its infancy. A number of good techniques exist based either on the physical or biological properties of the cells, and these have produced some valuable results. Still others are to be discovered. Therefore, the purpose of this open-end treatise is to acquaint the reader with some of the basic principles, instrumentation, and procedures pres- ently in practice at various laboratories around the world and to present some typical applications of each technique to particular biological prob- lems. To this end, I was fortunate to obtain the contribution of certain leading scientists in the field of cell separation, people who in their pioneer- ing work have struggled with the particular problems involved in separating living cells and in some way have won.