Word groups viewed as structurally and semantically inseparable units are traditionally regarded as the subject matter of phraseology. Phraseological units are usually defined as non-motivated word - groups that cannot be freely made up in speech but are reproduced as ready-made units. Phraseology as a linguistic discipline poses many problems regarding the meaning of phraseological units and their motivation. This article investigates the main sources from which appear phraseological units in the language and analyses phraselogical units according different criteria in the process of their adaptation to the needs of communication.