Formalizing UML; Mission Impossible? Peter van Emde Boas Abstract: Formalizing UML will lead to problems much larger than those encountered in formalizing ordinary computer related languages. These problems concern not primarily the syntax. In fact that part is most likely to be formalizable at all. Instead I expect major problems with specifying a semantics as long as its purpose is undecided. Also the fact that UML is a hybrid of languages rather than a single one will make life difficult. Finally the fact that UML is visual yields unusual problems in the pragmatics of the language.