ChemSpider is a free chemical structure database providing fast access to over 67 million structures, properties, and associated information. The data is supplied by various institutions and commercial companies and is curated on an ongoing basis by the staff and users to ensure data integrity and quality. ChemSpider has a wide user base, which includes students, teachers, lecturers, scientists, patent agents, publishers, and chemical manufacturers. By integrating and linking compounds from hundreds of high-quality data sources, it enables researchers to discover the most comprehensive view of freely available chemical data from a single online search. It builds on the collected sources by adding additional properties, related information, and links back to the original data sources.