The potential anticancer activities of histone deacetylase (HDAC) inhibitors and DNA methyltransferase (DNMT) inhibitors have been extensively studied in recent years. HDAC inhibitors suppress the activities of multiple HDACs, leading to an increase in histone acetylation. This histone acetylation induces an enhancement of the expression of specific genes that elicit extensive cellular morphologic and metabolic changes, such as growth arrest, differentiation and apoptosis. DNMT inhibitors, such as 5-aza-cytidine (5-aza-CR) and 5-aza-2'-deoxycytidine (5-aza-CdR) are also widely studied because DNA hypomethylation induces the re-activation of tumor suppressor genes that are silenced by methylation-mediated mechanisms. Recently, the combination of HDAC inhibitors or demethylating agents with other chemo-therapeutics has gained increasing interest as a possible molecularly targeted therapeutic strategy. In particular, the combination of HDAC inhibitors with demethylating agents has become attractive since histones are connected to DNA by both physical and functional interactions. To date, the accumulating evidence has confirmed the hypothesis that the combination of HDAC and DNMT inhibition is very effective (and synergistic) in inducing apoptosis, differentiation and/or cell growth arrest in human lung, breast, thoracic, leukemia and colon cancer cell lines. This review will discuss the in vitro effects of HDAC inhibitors, such as trichostatin A (TSA), sodium butyrate, depsipeptide (FR901228, FK228), valproic acid (VPA) and suberoylanilide hydroxamic acid (SAHA), and the demethylating agent, 5-aza-CdR used alone and in combination treatment of human cancer cells and the possible mechanisms involved.