This book defines the syntax, semantics, and restrictions you must follow to write valid XL Fortran programs. The compiler discovers most nonconformances of the XL Fortran language rules, but it may not find some syntactic and semantic combinations. This occurs because of either performance reasons, or because the nonconformance is only detectable at run time. XL Fortran programs that contain these undiagnosed combinations are not valid, whether or not they run as expected.