CFITSIO is a library of C and FORTRAN subroutines for reading and writing data files in FITS data format. CFITSIO simplifies the task of writing software that deals with FITS files by providing an easy to use set of high-level routines that insulate the programmer from the internal complexities of the FITS file format. At the same time, CFITSIO provides many advanced features that have made it the most widely used FITS file programming interface in the astronomical community.