This chapter discusses language proficiency in interpreting and opens with a discussion of several core concepts, including directionality, active languages, passive languages, working languages, and common designations for the languages with which interpreters work (e.g., L1, L2, and mother tongue). Then, measuring language proficiency is described more generally with the primary emphasis on Hulstijn’s BLC-HLC theory. General theories of language proficiency are then linked with those specific to interpreting, outlining different conceptualizations of language proficiency as well as the origins of the AIIC language classification. Empirical studies on language proficiency that expand on bilingualism studies scholarship are reviewed to delineate the characteristics of language proficiency necessary for interpreting. Finally, critical issues and topics, as well as emerging trends, in language proficiency in interpreting are discussed in order to promote a more nuanced understanding of language proficiency, both when screening candidates for interpreter education and when selecting research participants for future interpreting studies research.