Penn's collection of major reference works in chemistry is extensive and diverse, covering all topics from highly theoretical chemistry to energy topics that overlap with other fields. In recent years, Penn has been collecting chemistry reference works solely in electronic format, but the historic collection contains a hefty number of printed reference works The bulk of the collection is in organic and organometallic chemistry and provides researchers with overviews of different classes of compounds, their uses and behaviors, and methods of preparing them. Chemistry major reference works fall into three broad categories: databases of information about chemicals and reactions, encyclopedias, and comprehensive reviews of the literature. While individual encyclopedia entries are edited regularly to integrate the most up-to-date information, comprehensive sets are fixed in time, reviewing the literature during a particular time span, and new editions pick up where the previous set concluded.