HEASoft and XSPEC are now available as conda packages. See details ....

Historical Xspec Information

Details of design and implementation of Xspec v12.
An archive of the Xspec v11 manual.
Additional discussion about the conception of Xspec can be found in the Introduction to the Xspec Manual.
Background in Xspec as a component of the XANADU package.
Table of Xspec version numbers inside the HEASoft package.