Clauses and phrases are building blocks of language that are larger than phonemes, morphemes, and words and smaller than sentences and utterances. The clause is a group of words that usually contains a (finite) verb and, often, arguments of the verb. Clauses are the basic elements of what is called an utterance in oral language and a sentence in written language. This entry describes the role of clauses and phrases in the or- ganization of languages. It also discusses the acquisition of relative clauses, in various languages, in typically developing children and in children with language disorders and production and comprehension of complex syntax at the clause level in children with developmental language disorders. Utterances and sentences are considered complex if they contain two or more clauses, whereas simple utter- ances and sentences are made of one clause only. A phrase is a group of words that forms a meaningful unit within a clause. It is characterized as a grammatical unit at a level between the word and the clause and is usually built around verbs, nouns, adjectives, or adverbs. In some cases, the phrase can be a clause in itself or contain clauses and other phrases within it. The organization of phrases, clauses, utterances, and sentences varies from one language to another. It must be emphasized that the organization of grammar and the definition of a grammatical unit (including words) is difficult, and it is often impossible to provide neat boundaries between grammatical units. Thus, the descrip- tions in this entry are most appropriate for languages such as English and might in some cases be contradict- ed by very specific cases and constructions