Adult human beings have the ability to speak and understand at least one language. And while languages can vary considerably, all are intricate, complex, rule-governed systems. Humans use language with little or no conscious awareness of the underlying system(s) that they have (unconsciously) learned, and which enables them to communicate and interact with others. Linguistic theory seeks to understand the nature of these sytems: the sound systems (phonetics and phonology), the grammatical and meaning systems (syntax and semantics), and the interactions of these. The field addresses a variety of further questions including: How do these systems interact with communicative goals (pragmatics and discourse analysis). How are these systems acquired by children (child language acquisition). How do people actually produce and understand sentences in real time (language processing). What are the neural systems underlying speaking and understanding (neurolinguistics). How do the systems change over time, and how do these changes interact with and illuminate language structure (historical linguistics). How do people use these systems for social identity (sociolinguistics). How does language interact with culture (anthropological linguistics). Fields as diverse as anthropology, legal reasoning, language pathology, technical writing and editing, speech recognition, automatic machine translation, and natural language user interfaces all rely heavily upon methods and models developed in linguistics.