This volume of Methods in Enzymology includes up-to-date procedures used for the assembly of nucleosomes, chromatin, and nuclei, extending the classical procedures described nearly ten years ago in Volume 170 (Nucleosomes) of this series, and should assist in the further investigation of the ways in which the structural dynamics of chromatin contribute to the regulation of transcription, replication, recombination, and repair. Also described in this volume are assay for the structure and function of in vitro reconstituted chromatin and for defining the organization and characteristics of natural chromosomal material from yeast (Saccharomyces cerevisae), flies (Drosophila melanogaster), and frogs (Xenopus laevis), as well as from mammalian tissues. The purification and assay procedures for various chromatin remodeling activities, including histone acetyltransferases, histone deacetylases, and SWI/SNF ATPases are detailed. The critically acclaimed laboratory standard for more than forty years, Methods in Enzymology is one of the most highly respected publications in the field of biochemistry. Since 1955, each volume has been eagerly awaited, frequently consulted, and praised by researchers and reviewers alike. Now with more than 300 volumes (all of them still in print), the series contains much material still relevant today--truly an essential publication for researchers in all fields of life sciences.