The checksum keywords described here provide an integrity check on the information contained in FITS HDUs. (Header and Data Units are the basic components of FITS files, consisting of header keyword records followed by optional associated data records). The CHECKSUM keyword is defined to have a value that forces the 32-bit 1's complement checksum accumulated over all the 2880-byte FITS logical records in the HDU to equal negative 0. (Note that 1's complement arithmetic has both positive and negative zero elements). Verifying that the accumulated checksum is still equal to -0 provides a fast and fairly reliable way to determine that the HDU has not been modified by subsequent data processing operations or corrupted while copying or storing the file on physical media.