Since idiomatic expressions are strongly connected to the culture of specific languages, they are significant to teach and learn in English education in Swedish lower secondary school, as the communicative goals presented in Lgr22 reaches for students being able to participate in various social and cultural contexts. Previous research has revealed that material writers often design teaching materials based on intuition, which leads to less authentic language being taught. Therefore, this study investigates the quality of idiom teaching/learning materials included in two series of coursebooks. This was done by conducting a corpus study in the Contemporary Corpus of American English (COCA) and the British National Corpus (BNC) to reveal the frequency levels of the included idioms. Additionally, coursebook analyses were completed to examine the quality of the definitions and tasks concerning idioms. The findings revealed that the definitions of idioms in the coursebooks are vague, that idioms with no frequency in corpus are included, and that the chosen activities and exercises should be improved for them to fully support idiom acquisition. To help students reach the required proficiency level of Lgr 22, teachers need to be aware of these shortcomings when using coursebooks for idiom teaching.