Histones are a family of nuclear proteins that interact with DNA, resulting in DNA being wrapped around a core of histone octamer within the nucleosome. Acetylation/deacetylation of histones is an important mechanism that regulates gene expression and chromatin remodeling. Histone deacetylase (HDAC) inhibitors are a new class of chemotherapeutic drugs that regulate gene expression by enhancing the acetylation of histones, and thus inducing chromatin relaxation and altering gene expression. HDAC inhibitors have been shown in preclinical studies to have potent anticancer activities. A range of structurally diverse HDAC inhibitors have been purified as natural products or synthetically produced. Due to the promising preclinical activity of these agents, numerous clinical trials have been initiated. In this review, the results of published data of single agent and combination trials of these drugs are reviewed, with a focus on dosing, scheduling and toxicity. Although still early in drug development, there is a picture that is starting to develop as to the common toxicities and which tumors seem to be the most susceptible to this class of drugs.