Chromatin dynamics is a consequence of protein complexes that modify histones, remove histone modifications, mobilize nucleosomes or stabilize nucleosomes. A wide variety of such complexes have now been described. Some are abundant and may play global roles, others are more rare and specialized for functions at discreet loci. Several complexes share biochemical activities and genetic studies suggest overlapping functions. Many components of these complexes were first revealed in genetic screens, while others were discovered by novel biochemical approaches. This volume of Current Topics in Microbiology and Immunology reviews a wide variety of protein complexes that modify chromatin.