No child is born an English-speaker or a Mandarin-speaker. Upon birth, children learn the sound patterns, words, and sentence structures of English or of Mandarin, based on evidence from their environments. But what counts as evidence? Can young children learn a language merely from exposure to others using it, or must they be directly engaged to learn? Can they learn a language from television? Radio? Do they have to be paying attention? Is there a difference between “hearing” and “listening”? While answering these questions is critical for understanding language development—as well as how best to structure educational contexts for learners of all ages—the psychological literature offers no coherent picture. The seminar will read seminal studies alongside current empirical research, offering students an intensive introduction to language development theory and empirical methods, while also exploring open questions and active areas of research.