Jetbrains' Meta Programming System, recently released Open Source under Apache 2.0, is a language workbench for creating, extending and composing languages. A particular sweet spot is the incremental and modular extension of Java. This supports an incremental, bottom-up introduction of domain-specific abstractions where language extensions are added as a need for them arises in the project. The session explains how MPS works and the roles the different language extension aspects play. A running example is used to illustrate the concepts.