Histone deacetylases (HDACs; EC 3.5.1.98) are a class of enzymes that remove acetyl groups from an ε-N-acetyl lysine amino acid of histones. Inhibitors of this class of enzymes have a long history of use in psychiatry and neurology as mood stabilizers and anti-epileptics. Moreover, Histone deacetylase inhibitors (HDIs) are being studied as an alleviator or treatment for neurodegenerative diseases. Recently, this class of enzymes is emerging as an exciting new class of potential anticancer agents for the treatment of solid and hematological malignancies by inhibiting the proliferation and induction of differentiation and/or apoptosis of tumor cells in culture and in animal models. HDAC inhibition causes acetylated nuclear histones to accumulate in both tumor and normal tissues, providing a surrogate marker for the biological activity of HDAC inhibitors in vivo. HDAC inhibition not only results in acetylation of histones but also transcription factors such as p53, GATA-1 and estrogen receptor-alpha. The functional significance of acetylation of non-histone proteins and the precise mechanisms whereby HDAC inhibitors induce tumor cell growth arrest, differentiation and/or apoptosis are currently the focus of intensive research. Several HDAC inhibitors have shown impressive antitumor activity in vivo with remarkably little toxicity in preclinical studies.