Epigenetic modifications are reversible chromatin rearrangements that in normal cells modulate gene expression, without changing DNA sequence. Alterations of this equilibrium, mainly affecting the two interdependent mechanisms of DNA methylation and histone acetylation, are frequently involved in the genesis of cancer. The histone code, regulating gene expression, is constituted by the combination of different acetylated lysine residues of histones. In neoplastic cells, the abundance of deacetylated histones is usually associated with DNA hypermethylation and gene silencing. Several compounds, known to have in vitro antineoplastic activity, have been eventually shown to act as histone deacetylase inhibitors. Thus, HDAC inhibitors have been successfully introduced in clinical trials as antitumour agents. They are classified according to their chemical structures and are endowed with different specificity and affinity for the HDACs of classes 1, 2, 4. Among HDAC inhibitors, the most potent are the hydroxamic acid derivatives, like SAHA, which has been recently approved for therapy of cutaneous T-cell lymphomas. Other classes of HDAC inhibitors are short chain fatty acids (SCFA), benzamides, epoxyketone and non-epoxyketone containing cyclic tetrapeptides, and hybrid molecules. SCFA, although widely used (especially valproic acid) and clinically efficacious, have weak HDAC inhibition constants. Benzamides, like MS-275, and cyclic peptides, like depsipeptide, have been studied in numerous clinical trials and demonstrated low toxicity and activity in solid and haematological neoplasms. HDAC inhibitors are also potent radiation sensitizers. Their future in oncology may thus be based on their activity as single agents and on their synergy with the hypomethylating drugs and with chemo- and radiotherapeutics.