Adsorption chromatography depends upon interactions of different types between solute molecules and ligands immobilized on a chromatography matrix. The first type of interaction to be successfully employed for the separation of macromolecules was that between charged solute molecules and oppositely charged moieties covalently linked to a chromatography matrix. The technique of ion exchange chromatography is based on this interaction. Ion exchange is probably the most frequently used chromatographic technique for the separation and purification of proteins, polypeptides, nucleic acids, polynucleotides, and other charged biomolecules (1). The reasons for the success of ion exchange are its widespread applicability, its high resolving power, its high capacity, and the simplicity and controllability of the method.This handbook is designed as an introduction to the principles of ion exchange chromatography and as a practical guide to the use of the media available from Pharmacia Biotech. The handbook is illustrated with examples of different types of biological molecules which have been separated using ion exchange chromatography and different ways the technique can be used. For information on specific separations, the reader is recommended to consult the original literature.