In eukaryotes, the packaging of DNA by histones into chromatin has profound effects on transcription and other nuclear processes. Far from being a uniform set of 'beads on a string', chromatin structure is remodeled and even modified, altering the accessibility of packaged genes to the factors involved in their regulation. This special focus on chromatin features an Overview, six Reviews and a Perspective that describe how chromatin is built and subsequently altered at various structural levels, and the impact of these events on fundamental biological processes involving DNA.