A Java Virtual Machine or JVM, originally developed by Sun Microsystems, is a virtual machine that executes Java bytecode. This code is most often generated by Java language compilers, although the JVM has also been targeted by compilers of other languages.

The JVM is a crucial component of the Java platform. The availability of JVMs on many types of hardware and software platforms enables Java to function both as middleware and a platform in its own right. Hence the expression "Write once, run anywhere."

Starting with J2SE 5.0, changes to the JVM specification have been developed under the Java Community Process as JSR 924. As of 2006, changes to specification to support changes proposed to the class file format (JSR 202) are being done as a maintenance release of JSR 924. The specification for the JVM is published in book form, known as "blue book". The preface states:

We intend that this specification should sufficiently document the Java Virtual Machine to make possible compatible clean-room implementations. Sun provides tests which verify the proper operation of implementations of the Java Virtual Machine.
Kaffe is an example of a clean-room Java implementation. Sun retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Sun's specification.