The concept of the Current Header and Data Unit, or CHDU, is fundamental to the use of the CFITSIO library. A simple FITS image may only contain a single Header and Data unit (HDU), but in general FITS files can contain multiple Header Data Units (also known as `extensions'), concatinated one after the other in the file. The user can specify which HDU should be initially opened at run time by giving the HDU name or number after the root file name. For example, 'myfile.fits' opens the 5th HDU in the file (note that the numbering starts with 0), and 'myfile.fits[EVENTS] opens the HDU with the name 'EVENTS' (as defined by the EXTNAME or HDUNAME keywords). If no HDU is specified then CFITSIO opens the first HDU (the primary array) by default. The CFITSIO routines which read and write data only operate within the opened HDU, Other CFITSIO routines are provided to move to and open any other existing HDU within the FITS file or to append or insert new HDUs in the FITS file.