EMF Facet is a tool that allows non-intrusive extension of metamodels. It allows adding new attributes, operations, links, and subtypes without modifying the original metamodel. EMF Facet extensions are reusable across projects and do not require changes to existing models. The document provides examples of using EMF Facet to extend UML for SysML, extend Java metamodels for JPA and JUnit by adding new attributes and relations. EMF Facet is an open source Eclipse project initially contributed by MoDisco that provides customizable and computable extensions to metamodels.