Gary T. Leavens, Albert L. Baker, and Clyde Ruby Abstract JML is a behavioral interface specification language tailored to Java. It also allows assertions to be intermixed with Java code, as an aid to verification and debugging. JML is designed to be used by working software engineers, and uses Eiffel-style assertion syntax combined with the model-based approach to specifications typified by VDM and Larch. However, JML supports quantifiers, specification-only variables, frame conditions, and other enhancements that make it more expressive than Eiffel. This paper discusses the goals of JML, the overall approach, and describes the basic features of the language through examples. It is intended for readers who have some familiarity with both Java and behavioral specification using pre- and postconditions