The topic of my thesis is the treatment of idioms in English Lexicography. Its theoretical background and its practical consequences will also be presented. There are various types of dictionaries available in the market, which deal with idioms. One can found information on idioms even in general bilingual dictionaries, but there are specialised dictionaries too, which intended to describe idioms exclusively. However, certain dictionaries select a larger amount of idioms for description than others and the types of information they give about the lexical units may also vary. Furthermore, the language in which the explanation of headwords is provided can be the language of the phrase defined or another language. In the first case, the dictionary is monolingual, in the latter, it is bilingual. Of course, we could list more factors which differentiate dictionaries from each other. So knowing that there are many ways of presenting idioms in dictionaries and, therefore, there are many types of dictionaries dealing with idioms, I should have select dictionaries for analysis because it seemed to be impossible to give a detailed overview of each and every type of dictionaries which define and describe idioms.