Eukaryotic DNA is packaged into chromatin, a complex assembly of protein and nucleic acid. The histones within chromatin undergo extensive, highly regulated post-translational modification. One of the main functions of these modifications is to act as markers that ensure that the mutiprotein complexes that regulate the transcription, replication and repair of DNA are directed to the correct region of the genome at the appropriate time. This review focuses on recent biochemical and structural studies on how histones modified by acetylation, ubiquitination, phosphorylation and poly-ADP-ribosylation are recognized.