Histone deacetylase (HDAC) inhibitors are a new class of cancer chemotherapeutics in clinical development that target the family of enzymes that catalyze the removal of the acetyl modification on lysine residues of proteins, including the core nucleosomal histones H2A, H2B, H3 and H4. The balance of nucleosomal histone acetylation is maintained through the opposing actions of histone acetyltransferases (HATs) and HDACs, and plays an important regulatory role in gene transcription. Alterations in both HATs and HDACs have been identified in tumor cells and may contribute to the altered gene expression found in many cancers. Inhibitors of HDAC activity induce cell cycle arrest, differentiation or apoptosis in tumor cells, and inhibit tumor growth in a variety of rodent models of cancer. Several structurally diverse HDAC inhibitors have entered clinical trials and are demonstrating encouraging antitumor activity in a variety of cancer types. As we learn more about these enzymes and the biological processes that they regulate, a strong rationale is emerging for the development of HDAC inhibitors as anticancer agents.