Histone deacetylases (HDACs) are key enzymes in the regulation of gene expression. By maintaining the dynamic equilibrium of the acetylation status of highly conserved lysine residues on histones, they regulate chromatin remodeling and gene expression. A link between aberrant HDAC activity and cancer has been widely reported and HDAC inhibitors have been shown to inhibit the proliferation of human tumor cell lines in vitro. Furthermore, several HDAC inhibitors have exhibited potent anti-tumor activity in human xenograft models, suggesting this class of compounds to be promising novel cancer therapeutic agents. This review provides an update on the current knowledge of HDAC inhibition with a focus on the most recent progress of HDAC inhibitors in clinical development.