Language acquisition is the process by which humans acquire the capacity to perceive and comprehend language, as well as to produce and use words and sentences to communicate. Language acquisition is one of the quintessential human traits, because non-humans do not communicate by using language. Language acquisition usually refers to first-language acquisition, which studies infants’ acquisition of their native language. This is distinguished from second-language acquisition, which deals with the acquisition (in both children and adults) of additional languages. The capacity to successfully use language requires one to acquire a range of tools including phonology, morphology, syntax, semantics, and an extensive vocabulary. Language can be vocalized as in speech, or manual as in sign. The human language capacity is represented in the brain. Even though the human language capacity is finite, one can say and understand an infinite number of sentences, which is based on a syntactic principle called recursion. Evidence suggests that every individual has three recursive mechanisms that allow sentences to go indeterminately. These three mechanisms are: relativization, complementation and coordination. Furthermore, there are actually two main guiding principles in first-language acquisition, that is, speech perception always precedes speech production and the gradually evolving system by which a child learns a language is built up one step at a time, beginning with the distinction between individual phonemes.