In this thesis, I analyse the literary phenomenon of glossopoesis in Science Fiction texts from a frame of reference that regards fictional languages as active narrative constituents rather than unimportant or mere embellishment and perfumery, using a predominantly bibliographic method. An implication of this is that fictional languages should be the object of literary criticism and stylistic readings that can conflate fictional languages and storytelling instead of only highlighting properties such as linguistic accuracy or plausibility. After briefly defining underlying notions – literary glossopoesis and Science Fiction – I revisit Stockwell’s (2006) three broad functions of invented languages in light of Cheyne (2008) Tolkien (2016), and a few others to propose a new model made up of five specific functions: speculative, descriptive, rhetorical, diegetic and paratextual. While the first two functions seek to conciliate what Stockwell and Cheyne respectively refer to as ‘indexical’ and ‘alien encounter’ as well as ‘elaborative’ and ‘characterisation’, the rhetorical, diegetic and paratextual functions are concerned with the use of glossopoesis as rhetorical devices to generate readerly effects, diegetic tools that can mediate parallel, supporting or competing narratives, as well as paratextual or extradiegetic material, not written in prose and existing outside what is generally considered narrative whilst still having an impact on it. Using case studies with a corpus comprising of twentieth- and twenty-first-century English-language Science Fiction novels and short stories, I focus on texts in which fictional languages are narrative dominants to analyse them from the perspective of the narrative function that is most salient in each. The result will provide a critical model to approach narratives that feature glossopoesis in addition to new interpretative angles to both architexts and more recent stories.