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 differentiation are separated from one another, it will be exceedingly difficult to study some of the molecular mechanisms involved in cell recognition, spe­ cialization, interactions, cytotoxicity, and transformation. Clinical diagno­ sis of diseased states and use of isolated cells for therapeutic (e. g. , immu­ notherapy) 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-ended treatise is to acquaint the reader with some of the basic principles, instrumentation, and procedures presently in practice at various laboratories around the world and to present some typical applications of each technique to particular biological problems.