In language classrooms all over the world, textbooks are the most dominant teaching and learning materials. In these books, illustrations occupy a prominent place in teaching and learning processes. This qualitative study aims to investigate the frequency of occurrence and functions of illustrations in English course books. Four course books (5th, 6th 7th, and 8th grades) approved by the Ministry of National Education in Turkey were analyzed. In this qualitative research, content analysis was used as the research technique. In the analysis, Levin's 1981 typology including five categories in terms of the functions of illustrations (decorational, representational, organizational, interpretational, and transformational) was employed. Based on the results, it was found that most of the illustrations in four books were subsumed under the category of representational function. The results also showed that the books were mostly lacking transformational, interpretational, and organizational illustrations. This is considered a serious drawback of the books in terms of content-related illustrations since they make abstract concepts more memorable, enhance the recall of details of texts, and facilitate students' comprehension. The study also provides recommendations for the use of illustrations in English language course books.