Jess is a rule engine for the Java platform. To use it, you specify logic in the form of rules using one of two formats: the Jess rule language (prefered) or XML. You also provide some of your own data for the rules to operate on. When you run the rule engine, your rules are carried out. Rules can create new data, or they can do anything that the Java programming language can do.

Although Jess can run as a standalone program, usually you will embed the Jess library in your Java code and manipulate it using its own Java API or the basic facilities offered by the javax.rules API.

You can develop Jess language code in any text editor, but Jess comes with a full featured development environment based on the award-winning Eclipse platform.