To the best of our knowledge, the issue of idiomaticity is considered one of the greatest challenges in linguistics. It is due to the fact that English is an analytical language. Despite a wide range of idiomatic categories, the notion ‘idiom’ has been mostly used without pattern distinction. Consequently, the present article aims at examining the unpredictability of the lexical unit development and the idiomatic expression diffuse rule boundaries that lead to the diversified phenomenon of idiomaticity. Thus, to face the vast number and great diversity of idiom classifications, we are going to analyse how they have been categorized by different famous linguists who studied them in details. A.V. Koonin lays stress on the structural separateness of the elements in a phraseological unit and on the functions the units fulfil in speech. In his classification of V.V. Vinogradov developed some points first advanced by the Swiss linguist Charles Bally. N.N. Amosova’s approach is contextological. Prof. Smirnitsky classifies phraseological units according to the functional principle. Two groups are distinguished: phraseological units and idioms. Phraseological units are neutral, non-metaphorical when compared to idioms. Prof. Gvishiani’s classification: 1) word-combinations; 2) cliché-ed expressions; 3) phraseological units proper; 4) word-combinations with extra linguistic peculiarities; and 5) culture-bound word-combinations. As for the modern lexicology, Natalia Gvishiani analyses them according to the following semantic criteria: connotative and non-connotative word-combinations; 2) cliché-ed expressions; 3) phraseological units proper; 4) word-combinations with extra linguistic peculiarities; and 5) culture-bound word-combinations. Another idiom categorisation is thematically determined. It represents not only the subject matter but at the same time the cultural and sociolinguistic features, which are distinctive to a certain community. We intend to lay emphasis on the thematic clichés which have the British, American and Romanian components. In conclusion, it is important to mention the aim of this study.  There are a lot of attempts to classify the variety of idioms. In consequence, the outlined semantic, structural and last but not the least thematical criteria of idiom categorization should be deeply investigated to help the learner to concentrate on facets of complex lexical units and pin down the errors in phraseological usage. Thus the main purpose of the current article is to investigate the phenomenon of idiom categorization.