Write once, run anywhere (WORA), or sometimes Write once, run everywhere (WORE), was a 1995 slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. Ideally, this meant that a Java program could be developed on any device, compiled into standard bytecode, and be expected to run on any device equipped with a Java virtual machine (JVM). The installation of a JVM or Java interpreter on chips, devices, or software packages became an industry standard practice.

A programmer could develop code on a PC and expect it to run on Java-enabled mobile phones, as well as on routers and mainframes equipped with Java, without any adjustments. This was intended to save software developers the effort of writing a different version of their software for each platform or operating system they intend to deploy on.

This idea originated no later than the 1960s, with the IBM M44/44X, and in the late 1970s the UCSD Pascal system was developed to produce and interpret p-code. UCSD Pascal (along with the Smalltalk virtual machine) was a key influence on the design of the JVM, as is cited by James Gosling.

The catch is that since there are multiple JVM implementations, on top of a wide variety of different operating systems, there could be subtle differences in how a program executes on each JVM/OS combination, possibly requiring an application to be tested on each target platform. This gave rise to a joke among Java developers: Write Once, Debug Everywhere.

In comparison, the Squeak Smalltalk programming language and environment boasts of being truly write once run anywhere, because it runs bit-identical images across its wide portability base.

Like Squeak, MicroEJ provides a virtual execution environment that guarantees one unique Java thread policy across all implementations, ensuring a true WORA semantic across millions of devices.