The tails of the four core histones are exposed on the nucleosome surface, where they are subject to a variety of enzyme-catalyzed, post-translational modifications. Modifications, singly or in combination, provide a source of information that can be used for signal transduction during ongoing processes, such as transcription, or as heritable epigenetic marks. A nomenclature is presented that allows patterns of histone modification to be clearly and unambiguously specified and that should facilitate discussion of their functional roles.