Post translational modification of histones and non-histone proteins by acetylation play a key role in tumourigenesis. Histone deacetylases (HDACs) are enzymes involved in remodelling of chromatin by deacetylating the lysine residues and play a pivotal role in epigenetic regulation of gene expression. An aberrant activity of HDACs has been documented in several types of cancers and HDACs have emerged as an attractive therapeutic target. HDAC inhibitors (HDACi) are a structurally diverse group of anti-cancer agents which have a potential role in regulation of gene expression and induction of cell death, cell cycle arrest, and differentiation by altering the acetylation status of histone and non-histone proteins. HDACi have pleiotropic effects on malignant cells and have demonstrated potent anti-cancer activity in pre-clinical studies. A number of clinical trials of HDACi as a monotherapy and/or in combination with conventional and novel chemotherapeutic drugs in solid and haematologic tumours have been published with variable efficacy.