The Chandra X-ray Observatory is the U.S. follow-on to the Einstein Observatory and one of NASA"s Great Observatories. Chandra was formerly known as AXAF, the Advanced X-ray Astrophysics Facility, but renamed by NASA in December, 1998. Originally three instruments and a high-resolution mirror carried in one spacecraft, the project was reworked in 1992 and 1993. The Chandra spacecraft carries a high resolution mirror, two imaging detectors, and two sets of transmission gratings. Important Chandra features are: an order of magnitude improvement in spatial resolution, good sensitivity from 0.1 to 10 keV, and the capability for high spectral resolution observations over most of this range. The Chandra Source Catalog (CSC) includes information about X-ray sources detected in observations obtained using the Chandra X-ray Observatory. Release 2.1 of the catalog includes 407,806 point, compact, and extended sources detected in ACIS and HRC-I imaging observations released publicly prior to the end of 2021. Observed source positions and multi-band count rates are reported, as well as numerous derived spatial, photometric, spectral, and temporal calibrated source properties that may be compared with data obtained by other telescopes. Each record includes the best estimates of the properties of a source based on data extracted from all observations in which the source was detected. The Chandra Source Catalog is extracted from the CXC"s Chandra Data Archive (CDA). The CXC should be acknowledged as the source of Chandra data. For detailed information on the Chandra Observatory and datasets see: http://cxc.harvard.edu/ for general Chandra information; http://cxc.harvard.edu/cda/ for the Chandra Data Archive; http://cxc.harvard.edu/csc/ for Chandra Source Catalog information.