This book will teach you how to write programs in the REXX language on IBM mainframe computers. It is useful for technical, applications, and maintenance programmers working on new or legacy code written in the REXX language. It describes the REXX programming language for TSO, MVS, OS/390 and Z/OS. (IBM mainframe operating system environments). The book quickly gets you writing programs in REXX. It covers all REXX language elements, all REXX built-in functions, and the more important TSO-only functions. You will find URLs in the book linking to a set of practice problems on a public webpage, as well as additional information on setting up your TSO session to execute REXX programs. This book was written and published in 2012 by the author of "REXX in the TSO Environment." The REXX Language on TSO improves on "REXX in the TSO Environment" in every way: it clarifies important points; it is better organized, and gives better examples. REXX is a high-level scripting language created in the 1980's by IBM for the IBM mainframe operating systems CMS and MVS. It quickly gained popularity and expanded into other environments such as Netview, CICS, AIX, QMF, VSE and OS/2. REXX is often used for prototyping applications, front-ending operating system commands and TSO/ISPF panel applications. Its flexibility and clear straightforward syntax makes it a welcome replacement for IBM utility programs. REXX is generally not used for high-volume business applications because of its performance limitations with respect to compiled or assembled languages. REXX is able to read and write sequential files and/or PDS members. It contains all programming language features, including the ability to do calculations with decimal numbers to any precision. It is especially useful in manipulating character strings in free-column format. There is a chapter containing examples of REXX programs, macros, and user-written functions. Explanations of the code are shown on the facing page.