Over the past 10 years high performance liquid chromatography (LC) has evolved into a mature analytical technique. Most LC separations are currently carried out in the reversed-phase mode using 25-cm columns filled with 5 or 10 μm nonpolar bonded substrates. A smaller fraction of separations are carried out in the normal-phase mode with polar bonded phases. Although major changes in column technology have become less commonplace in recent times, refinements continue to be made. New bonded phases have been developed for the separation of specific classes of compounds and new substrates have been introduced that extend column lifetimes and permit use over wide pH intervals. Even though bonded phase usage is now common, solute retention mechanisms with these materials are not fully understood (but considerable progress has been made). To aid the study of retention mechanisms, novel approaches have been taken in the physical and chemical characterization of bonded phase sorbents.