One of the striking differences between a human translator and any MT system is the fact that a human translates propositions rather than sentences or eve nwords. Thus, when translating, a human abstracts from the formal structure of a sentence and concentrates on its propositional content. Unlike human translators, computers do not understand the texts they translate. This absence or lack of understanding in MT systems is called the semantic barrier. This paper briefly discusses some aspects of ambiguity resulting from the semantic barrier and their consequences for MT systems.