Histone deacetylases (HDACs) remove acetyl groups from the tails of lysine residues of histone protein in nuclear chromatin and also from acetylated sites in non-histone proteins. HDACs and histone acetyltransferases (HATs) are major influences on the level of cellular protein acetylation, and an imbalance in acetylation levels, particularly under-acetylated (hypoacetylated) histone protein has been associated with precancerous or malignant states. Consequently, small molecule inhibitors of HDACs have been synthesised and some now form a newly emerging class of anti-cancer agents that can regulate transcription and inhibit proliferation of cancer cells by inducing cell cycle arrest, differentiation and/or apoptosis, among other major biological phenomena. The different mechanism(s) of action of HDAC inhibitors compared to conventional anti-neoplastic agents provides a possibility that HDAC inhibitors may be effective for refractory cancers. Accordingly, a number of programs for the development of HDAC inhibitors as anti-cancer drugs have been initiated. This review highlights recent developments in the design, synthesis and biological properties of HDAC inhibitors in the context of potential cancer therapy.