Roughly half of the world’s population are bilingual, that is, around four billion people. Worldwide, language learning is on the rise, driven by factors such as immigration, globalization, and an increased awareness of the value of learning another language. In this chapter we explain how we learn languages in addition to our mother tongue, that is, the language we grew up speaking from early childhood. How is learning a second language different to learning a first? What are some of the challenges people face when learning another language? We explore issues around translation, and the creative inventions of sci-fi like the babel fish and the Tardis, versus the capabilities and limitations of AI. We take a look at unique cases of true (and fake) polyglot savants, and we revisit those who suddenly speak with another accent, or even in an entirely different language. We also see what science says about the considerable cognitive and social benefits of learning a new language.