Chromatin composition differs across the genome, with distinct compositions characterizing regions associated with different properties and functions. Whereas many histone modifications show local enrichment over genes or regulatory elements, marking can also span large genomic intervals defining broad chromatin domains. Here we highlight structural and functional features of chromatin domains marked by histone modifications, with a particular emphasis on the potential roles of H3K27 methylation domains in the organization and regulation of genome activity in metazoans. Chromatin domains are extended genomic (...) regions defined by the continuous enrichment of a given histone modification. Here, we review recent work highlighting the presence of chromatin domains in multiple species, their structural aspects in the context of chromatin architecture, and their potential role on the regulation of genome activity. (shrink)