In our increasingly multilingual modern world, understanding how languages beyond the first are acquired and processed at a brain level is essential to design evidence-based teaching, clinical interventions and language policy. Written by a team of world-leading experts in a wide range of disciplines within cognitive science, this Handbook provides a comprehensive overview of the study of third (and more) language acquisition and processing. It features 30 approachable chapters covering topics such as multilingual language acquisition, education, language maintenance and language loss, multilingual code-switching, ageing in the multilingual brain, and many more. Each chapter provides an accessible overview of the state of the art in its topic, while offering comprehensive access to the specialized literature, through carefully curated citations. It also serves as a methodological resource for researchers in the field, offering chapters on methods such as case studies, corpora, artificial language systems or statistical modelling of multilingual data.