The construct of Multilingualism with English has been acknowledged in the field of ELF and, to some extent, multilingualism in applied linguistics. Implying the encompassing position of multilingualism (Jenkins, 2015), the construct has come to refer to multilingual repertoires in which English takes one part. In this sense, it differs from cases of multilingualism in which English does not exist in ML repertoires. English in this construct, meanwhile, implies ELF by definition. However, this does not mean that native English speakers’ multilingualism is out of the scope of this definition, either. Interactions involving multilingual native and non-native speakers can be placed in this category, as well. Against this background, Multilingualism with English will be described in this entry through its ontology and epistemology, two major pillars of scientific inquiry.