The high-mobility group (HMG) proteins are a small set of distinctive nonhistone chromatin (NHC) proteins which, without doubt, have been studied more intensively than any other nonhistones. That is reflected by the inclusion in this volume of several chapters that deal with the HMG proteins. The attention that has been focused on these proteins is in large part due to practical considerations. After the histones, the HMG proteins are the chromatin proteins most amenable to investigation with currently available techniques. They are among the most abundant nonhistone proteins of chromatin, they are readily extracted from chromatin, and are soluble under nondenaturing conditions. Hence, the HMG proteins are relatively easy to purify, in comparison to other NHC proteins. Further, the acid solubility of the HMG proteins allows them to be easily identified in complex mixtures of proteins with at least some degree of confidence. As a result of the comparative ease with which they can be studied, a great deal of structural information has been accumulated on HMG proteins. In addition, studies aimed more directly at elucidating their functions have raised intriguing possibilities for the biological roles of the HMG proteins, with the effect of greatly heightening interest in them. That interest is fully justified. At the same time, there is no reason to believe that many other NHC proteins will be any less interesting when they have been investigated as thoroughly as the HMG proteins.