A key event in the regulation of eukaryotic gene expression is the posttranslational modification of nucleosomal histones, which converts regions of chromosomes into transcriptionally active or inactive chromatin. The most well studied posttranslational modification of histones is the acetylation of ϵ‐amino groups on conserved lysine residues in the histones' amino‐terminal tail domains. Significant advances have been made in the past few years toward the identification of histone acetyltransferases and histone deacetylases. Currently, there are over a dozen cloned histone acetyltransferases and at least eight cloned human histone deacetylases. Interestingly, many histone deacetylases can function as transcriptional corepressors and, often, they are present in multi‐subunit complexes. More intriguing, at least some histone deacetylases are associated with chromatin‐remodeling machines. In addition, several studies have pointed to the possible involvement of histone deacetylases in human cancer. The availability of the cloned histone deacetylase genes has provided swift progress in the understanding of the mechanisms of deacetylases, their role in transcription, and their possible role in health and disease. J. Cell. Physiol. 184:1–16, 2000. © 2000 Wiley‐Liss, Inc.