The eukaryotic DNA is invariably in the form of a nucleoprotein complex called chromatin. The larger protein fraction in chromatin is made up of small basic proteins called histones. In the past the histones were generally considered the repressors of the transcriptional and replicative processes. Present opinion stresses their structural role in organizing the DNA molecules in chromatin fibres. The association of histones with DNA is not random and leads to an orderly, repetitive structure. Analysis by electron microscopy and by several other optical and biochemical techniques have demonstrated that chromatin is organized in subunits which have been called “P.S.” by Rill and Van Holde (1973), “v” bodies by Olins and Olins (1973, 1974) and nucleosomes by Oudetet al. (1975). These subunits are formed by an octamer of two molecules of each of the histones H2a, H2b, H3 and H4, associated with a string of 140 base pairs of DNA (which is especially resistant to nuclease digestion). The subunits are linked by a DNA segment (whose length varies in different cell types) associated with histone H1. The internal structure and spatial arrangement of the nucleosomes, their possible association with nonhistone proteins and the significance of the nucleosomal structure in chromatin during the replicative and transcriptional processes are discussed.