A panel of leading investigators summarizes and synthesizes the new discoveries in the rapidly evolving field of histone acetylation as a key regulatory mechanism for gene expression. The authors describe what has been learned about these proteins, including the identification of the enzymes, the elucidation of the enzymatic mechanisms of action, and the identification of their substrates and their partners. They also review the structures that have been solved for a number of enzymes-both alone and in complex with small molecule inhibitors-and the biological roles of the several histone deacetylases (HDAC) genes that have been knocked out in mice.