A key feature of cancer is dysregulation of gene activity and gene expression, which is driven by a combination of acquired genetic and epigenetic alterations. Here, we will highlight how insights into the epigenetic processes underpinning tumor biology have led to the emerging field of cancer pharmaco-epigenomics. First, we will discuss how interference with the epigenetic machinery in cancer is leading to novel promising therapies, with several DNA methyltransferase and histone deacetylase inhibitors being approved for cancer treatment. Secondly, we will discuss how epigenetic markers in cancer may increasingly be used as complementary diagnostic tools, prognostic markers of disease progression and predictive markers of treatment response. Although the anti-tumoral activities of epigenetic therapies have thus far been attributed to reactivation of silenced tumor suppressor and/or apoptotic genes, they may also influence the tumor environment by directly affecting stromal cells. As an example, we will discuss how tumor endothelial cells are regulated at the epigenetic level and are affected by methyltransferase and histone deacetylase inhibitors.