When we speak, we spontaneously gesture with our hands. But these gestures are not anecdotal, superfluous “illustrations” of what is being said. Instead, they are thought to be part and parcel of language. In other words, gestures and speech are assumed to form an integrated system. Furthermore, gestures can facilitate language comprehension in monolingual and multilingual settings. Our particular interest is in simultaneous interpreting, a multilingual setting involving concurrent language comprehension and production in two languages. As such, it is one of the most complex language tasks. But what role do speakers’ gestures play for interpreters during simultaneous interpreting? This question is the focus of this thesis. We report on empirical work investigating (1) whether speakers’ gestures facilitate language comprehension during simultaneous interpreting; (2) whether speakers’ gestures facilitate language comprehension during simultaneous interpreting in degraded listening conditions; and (3) whether gestures and speech mutually and obligatorily interact during language processing in simultaneous interpreting.