This chapter is dedicated to discussing the engineering aspects involved in the integration of modeling languages, as an essential part of the globalization process. It covers the foundations of language integration, the definition of the relationships between the languages to be integrated, and the various dimensions of language and tool integration. Language variants, evolution, refactoring and retirement are also discussed, as key issues involved in the globalization of modeling languages. Today's software development is characterized by a large degree of improvisation: di↵erent languages and language-based tools are used to create di↵erent artifacts that act on the same (physical or logical) system. Such multiplicity is not a problem in itself, but rather the common situation of those languages and tools to be insuciently integrated. The languages and tools though involve interaction at the system level and these interactions are hard to understand without good integration. In fact, the actual interactions may disagree with the intended interactions. Any analysis or verification is going to be hard. Excessive testing may be needed and still fail to be conclusive. We just may hope for the engineers collaborating on the same system to limit themselves to a set of languages implemented with a single language workbench. Version changes of languages and tools would be semantics preserving (both backwards and forward compatible) and therefore not require any changes to the current artifacts.