This paper deals with the comparison and analysis of idioms occurring in English students' books designed for students at lower-secondary schools and for adult beginners. The aim of the thesis is to search for and examine the most common idioms, their usage in exercises and the difference between their presence in students' books for teenagers and adults at the same level of their language skills. Firstly, the linguistic analysis of idioms and general division of language skills is looked into, secondly, the thesis is concerned with the importance of a corpus and its possible utilisation for teachers. The second, practical part of the thesis focuses on the didactic aspect, i.e. the way idioms are presented and practiced in coursebooks and their presence among exercises aimed at different language skills. Finally, this paper compares the frequency of occurrence of found idioms between coursebooks and a corpus.