Transformed cells, characterised by inappropriate cell proliferation, do not necessarily lose the capacity to undergo growth arrest under certain stimuli. DNA, genetic information, is packaged in chromatin proteins, for example, histones. The structure of chromatin may be altered by post-translational modifications (e.g., acetylation, phosphorylation, methylation and ubiquitylation) which play a role in regulating gene expression. Two groups of enzymes, histone deacetylases (HDACs) and acetyl transferases, determine the acetylation status of histones. This review focuses on compounds that inhibit HDAC activity. These agents have been shown to be active in vitro and in vivo in causing cancer cell growth arrest, differentiation and/or apoptosis. Several HDAC inhibitors are currently in clinical trials as anticancer agents and, in particular, hydroxamic acid-based HDAC inhibitors have shown activity against cancers at well-tolerated doses.