"Write once, run anywhere" (WORA), or sometimes Write once, run everywhere (WORE), is a slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. Ideally, this means Java can be developed on any device, compiled into a standard bytecode and be expected to run on any device equipped with a Java virtual machine (JVM).

The catch is that since there are multiple JVM implementations, on top of a wide variety of different operating systems such as Windows, Linux, Solaris, NetWare, HP-UX, and Mac OS, there can be subtle differences in how a program may execute, which may require an application to be tested on various target platforms. This has given rise to the joke among Java developers, "Write Once, Debug Everywhere".