Idioms, because of their cultural and figurative aspects, cannot be readily comprehended and used appropriately; hence, they need to be taught explicitly by means of instructional materials in language classrooms. Knowledge of idioms constitutes an important component of learners' communicative competence (Bachman, 1990) since idioms as part of cultural references, and figurative, functional and formulaic language can help to enhance not only organizational competence but also pragmatic competence. Therefore, the aim of this study is to investigate EFL teacher-trainees' perceptions of their idiom knowledge, attitudes towards idiom instruction and their use of idioms in in-house instructional materials. In doing so, the materials designed by EFL teacher-trainees were examined by means of content analysis followed by the analysis of questionnaire and semi-structured interview data. The quantitative and qualitative data analyses show that the teacher-trainees hardly used idioms in their materials and those who used them usually did not incorporate them in a way to help enhance learners' communicative competence. The data analysis also reveals that teacher-trainees' insufficient idiom learning experience in the past could have been influential in their use of idioms in the materials, although they did show positive attitudes towards idiom instruction. The study has important implications for teacher education, which should emphasize the role of culture-specific expressions in developing future teachers' language as well as pedagogic competence.