Chromatin Immunoprecipitation (ChIP) is an invaluable method for studying the interactions between proteins and DNA on a genome-wide scale. ChIP can be used to determine whether a transcription factor interacts with a candidate target gene, and is used to monitor the presence of histones with posttranslational modifications at specific genomic locations. The results are often extremely useful for investigating the functions of specific transcription factors or histone modifications. In the September issue of Cold Spring Harbor Protocols, Michael Carey, Craig Peterson and Stephen Smale present Chromatin Immunoprecipitation (ChIP), an optimized protocol for use in mammalian cells. This is one of September’s featured articles, and like all our featured articles, it is freely available to subscribers and non-subscribers alike.