Methylation of DNA at cytosine residues as well as post-translational modifications of histones, including phosphorylation, acetylation, methylation and ubiquitylation, contribute to the epigenetic information carried by chromatin. These changes play an important role in the regulation of gene expression by modulating the access of regulatory factors to the DNA. The use of a combination of biochemical, genetic and structural approaches has allowed demonstration of the role of chromatin structure in transcriptional control. The structure of nucleosomes has been elucidated and enzymes involved in DNA or histone modifications have been extensively characterized. Since deregulation of epigenetic marks has been reported in many cancers, a better understanding of the underlying molecular mechanisms bears the promise that new drug targets may soon be found. The newest developments in this quickly developing field are presented in this book.