This is the first in a series of volumes concerning the properties of the eukaryotic nucleus. Contributions from several of the most active laboratories are brought together to present a focused overview of a selected aspect of nuclear structure and function. Research on the role of nucleosomal structures in transcriptional regulation is making rapid progress. Experimental work on the accessibility of genes to the transcriptional machinery has clearly established that chromatin structure has a role in the repression of genes. However, this repression is highly selective, dependent upon the formation of specific nucleosomes that contain key regulatory elements. More importantly, genetic analysis and biochemical reconstitution have suggested that nucleosomal structures have an essential role in the activation of transcription, and that in certain instances can actually facilitate the inititation process.