Humans acquire language naturally, namely without specific instruction, by being exposed to it and by interacting with other human beings. According to the generativist enterprise, humans are endowed with a system of knowledge on the form of possible human languages (Universal Grammar). Evidence consistent with this assumption is provided in the chapter, by illustrating crucial phenomena ranging from the acquisition of phonology, to morphosyntax, syntax, formal semantics, and pragmatics. Infants’ brain organization is tuned to speech stimuli and presents left hemisphere specialization, from the first days of life, if not already in the mother’s womb. Infants set apart languages at two days of age, based on durational or rhythmic properties. Toddlers combine words by respecting the basic word order and are very sensitive to the hierarchical organization of sentences, both when it comes to the syntactic structure and when it comes to the interpretation of sentences.