UML Profile development is handled by the J language, an object language similar to Java and very easy to implement. J is a dynamic and interpreted object language, dedicated to browsing in the metamodel and to building model transformation rules. Other than the notions of generalization, attributes and operations, J also provides the concepts of context, browsing and diffusion on a set of objects. These constitute a decisive factor, both in terms of power and in terms of simplicity. J rules can be completed, redefined, overloaded, with all the power of an object language. They are structured into profiles and can be easily adapted by the user.