The relationship between multilingualism and autism is still a vastly under-explored topic. The research that has so far been published shows a positive relationship between multilingualism and cognitive abilities in autism (Gonzalez-Barrero & Nadig, 2018; Uljarevi ́c, Katsos, Hudry, & Gibson, 2016), replicating previous research on non-autistic children and adults (Bialystok, 2011). Furthermore, when surveyed, autistic adults have responded positively about learning multiple languages (Digard, Sorace, Stanfield, & Fletcher-Watson, 2020). However, there is no research that currently captures the choices autistic adults make when using and switching between their languages. Furthermore, even though positive relationships between multilingualism and cognition have been established, how multilingual language usage relates to autism is still poorly understood. Therefore, this thesis seeks to answer the following questions: • What multilingual language choices do autistic adults make, compared to their non- autistic counterparts? • Does multilingual language use predict differences in autistic traits, both in autistic children and in autistic and non-autistic adults? The first question is answered in Chapter 6, by investigating the self-reported multilingual language usage of autistic adults, compared to non-autistic adults. Differences between autistic and non-autistic participants in this chapter included, for example, the frequency with which they choose to switch between languages with different interlocutors and the self-reported effort of code-switching, reported by autistic adults as more strenuous than non-autistic adults. Chapters 2 to 5 and 7 answer the second research question by examining the association be- tween language usage on autistic traits in autistic children and autistic and non-autistic adults across five separately collected data-sets. Chapters 2 to 5 and 7 conclude that a higher degree of multilingual usage associates with lower autistic traits in autistic children and non-autistic adults but not in autistic adults. By using multinational data from multiple, independently collected sources, this thesis provides substantive evidence of a relationship between multilingual language usage in autistic traits. Therefore, this research presents a significant contribution to current understandings of multilingualism and autism. Finally, as outlined in Chapter 8, the findings in this thesis lay the foundation for future investigations into the relationship between multilingual language usage and autism.