Idioms represent a fascinating linguistic phenomenon that has captured the attention of many linguists for decades. This corpus-based study of idioms in Modern Standard Arabic sheds light on their intricate nature, establishes the major patterns of their linguistic behaviour, and provides explanations for these patterns. Adopting a descriptive framework, the study addresses two main issues with regard to the discursive behaviour of idioms: the discursive functions that they perform and the ways in which they contribute to the cohesion of their texts. Examining primary data with regard to the semantic, discursive, lexical and grammatical properties of Arabic idioms, the author touches on the ubiquity of these expressions in language use, the wide range of functions they perform in discourse, the problems they often cause in domains such as foreign language learning and translation, and their typical divergence from the normal rules of grammar and semantic compositionality. Providing explanations for major linguistic phenomena, this analysis will be accessible to linguists, translators, lexicographers, translation software developers, and language teachers as well as learners.