ChemSpider is a free chemical structure database providing fast access to over 67 million structures, properties, and associated information. By integrating and linking compounds from hundreds of high-quality data sources, ChemSpider enables researchers to discover the most comprehensive view of freely available chemical data from a single online search. It is owned by the Royal Society of Chemistry. ChemSpider builds on the collected sources by adding additional properties, related information, and links back to original data sources. ChemSpider offers text and structure searching to find compounds of interest and provides unique services to improve this data by curation and annotation, and to integrate it with users’ applications.