The main structural features of glycogen were established many years ago by predominantly chemical methods. However, the fine structure continues to be the subject of experimental investigations by enzymic methods. Current topics include: (a) the mechanism of the formation of the linkage between glycogen and protein during the first stages of biosynthesis; (b) the arrangement of the constituent chains to form ß-particles of molecular weight ~ 107; (c) the mode of aggregation of ß-particles to form much larger a-particles; and (d) the possible presence of monosaccharide residues other than d-glucose, and of phosphate ester groups. This review comprises a critical discussion of the above and related aspects of glycogen structure. © 1991.