The ZCAT database contains the CfA Redshift Catalog, which incorporates much of the latest velocity data from the Whipple Observatory and other sources, as well as velocities from earlier compilations such as the "Second Reference Catalog" of de Vaucouleurs, de Vaucouleurs, and Corwin; the "Index of Galaxy Spectra" of Gisler and Friel; and the "Catalog of Radial Velocities of Galaxies" of Palumbo, Tanzella-Nitti, and Vettolani. It includes BT magnitudes, some UGC numbers, and increased "accuracy" in the velocity source information. The data presented here have primarily been assembled for the purpose of studying the large scale structure of the universe, and, as such, are nearly complete in redshift information, but are not necessarily complete in such categories as diameter, magnitude, and cross-references to other catalogues. The original HEASARC version was constructed based on an earlier version of the catalog and was released on November 15, 1996. The HEASARC created the current version of ZCAT in February 2001 based on CDS/ADC Catalog VII/193, "The CfA Redshift Catalogue", Version June 1995, tables <a href="https://cdsarc.cds.unistra.fr/ftp/cats/VII/193/zcat.dat">https://cdsarc.cds.unistra.fr/ftp/cats/VII/193/zcat.dat</a> and zbig.dat. The former table contains the main body of the CFA Redshift Catalog (57536 objects) and entries from it are distinguishable in the current database by having either listed radial velocity values but not redshifts or neither, while the latter table contains 1202 high-redshift galaxies (distinguishable in the current database by their having listed redshift values but not radial velocities). This is a service provided by NASA HEASARC .