​To faithfully execute diverse biological programs all cells need to access and distribute their genomes in a highly organized way. In the nucleus of eukaryotic cells DNA is packed with histone proteins into chromatin. The originating nucleo-protein complex is the regulatory platform for all genetic processes. Of these, posttranslational modifications of the histone proteins play a key role as they are thought to direct different chromatin states. Most histone modifications appear to not have a direct effect onto chromatin structure, but work via recruitment of specific binding proteins. A large number of such individual factors interacting with diverse histone marks have been identified and characterized. Also, global approaches have been established that aim to define the interactome of histone modifications or patterns thereof. We summarize the experimental approaches that are used to determine histonemodification readout and discuss complexities that are emerging within this regulatory system.​