The object of this Review is to give analytical chemists a general introduction to molecular-sieve chromatography, a form of liquid chromatography in which molecular size forms the primary basis for separation, although other effects are also frequently involved. The technique can be used for inorganic and organic molecules, both monomeric and polymeric, in either aqueous or non-aqueous systems. The range of xerogel and aerogel molecular-sieves available at present is described, and the experimental techniques involved in their use are emphasized rather than mechanistic and theoretical considerations. The references cited have been selected critically to form a balanced, up-to-date review and also to indicate the general analytical potential and scope for future development of the technique. An Appendix lists the commercial sources of molecular-sieves and calibration standards.