Post-translational modifications are a general mechanism for instructing proteins about their function. Moreover, they allow proteins to “memorize” their encounters, such as contacts with other proteins. Histone proteins are the key examples of this protein communication system. In particular, lysine acetylations and methylations of histone tails have a large functional impact. Genome-wide profiling of the large set of post-transcriptional histone modifications provides the basis of the histone code. This code leads to the understanding, how the epigenome directs transcriptional regulation and stores information.