What sorts of alien languages can there be? And might reality be such that some alien language represents reality better than familiar languages do? Alien languages are here languages that use different kinds of semantic tools than any familiar languages use. The question of the existence of alien languages is interesting in itself: what kinds of languages are possible? But attending to the issue of alien languages also problematizes the relationship between language and reality, and highlights the possibility that reality could have a fundamentally different structure than we otherwise take it to have. Despite the foundational significance of these questions, they have received virtually no explicit attention in the literature. But the book brings up and criticizes existing contemporary work that promises to be at least indirectly relevant. A main claim of the book is that alien languages are possible and that we should be alert to the possibility that reality has alien structure and is best described by an alien language. The book also raises other possibilities regarding this debate. Maybe we should not say that the world has either kind of structure but can be equally well described using either kind of language.