When operating systems are developed to meet the needs of the computing marketplace, applications are written to run on those operating systems. Over the years, many applications have been developed that run on z/OS® and, more recently, UNIX®. To accommodate customers with UNIX applications, z/OS contains a full UNIX operating system in addition to its traditional z/OS interfaces. The z/OS implementation of UNIX interfaces is known collectively as z/OS UNIX System Services, or z/OS UNIX for short.

The most common interface for z/OS developers is TSO/E and its panel-driven interface, ISPF, using a 3270 terminal. Generally, developers use 3270 terminal emulators running on personal computers, rather than actual 3270 terminals. Emulators can provide developers with auxiliary functions, such as multiple sessions, and uploading and downloading code and data from the PC.

Program development on z/OS typically involves the use of a line editor to manipulate source code files, the use of batch jobs for compilation, and a variety of mechanisms for testing the code. Interactive debuggers, based on 3270 terminal functions, are available for common languages. This section introduces the tools and utilities for developing a simple program to run on z/OS.

Development using only the z/OS UNIX portion of z/OS can be through Telnet sessions (from which the vi editor is available) through 3270 and TSO/E using other editors, or through X Window System sessions from personal computers running X servers. The X server interfaces are less commonly used.

Alternate methods are available in conjunction with various middleware products. For example, the WebSphere® products provide GUI development facilities for personal computers. These facilities integrate TCP/IP links with z/OS to automatically invoke mainframe elements needed during development and testing phases for a new application.