This dissertation explores multilingual language control (LC) in the context of simultaneous interpreting (SI) using a two-pronged approach. First, language variety co-activation patterns in comprehension in trained interpreters and untrained multilinguals (N=64; L1=German; L1' Swiss German for half of the participants) are investigated in a Visual World eye-tracking study and compared to co-activation in a bilingual comprehension-and-production task. Between-group differences are found in comprehension, however, they correlate with language-variety use and are not attributable to SI training. Second, the potential impact of SI training on LC abilities, as well as the extent to which LC overlaps with executive control (EC) is investigated longitudinally. The performance of a group of SI trainees is compared to that of translation trainees (n=14 per subgroup) before and after one year of SI training. No between-group performance differences emerge in the pre-test, and EC-task performances are unaffected by SI training. Regarding LC, the findings suggest domain-specific SI-training related changes, but do not point towards an SI-induced cognitive advantage.