This study aims at diagnosing which subjects pre-service physics teachers have difficulty understanding in introductory physics courses and what accounts for these difficulties. A questionnaire consisting of two qualitative questions was used to collect data for this study. The questionnaire was administered to 101 pre-service physics teachers who have completed the courses Physics 1 (Mechanics 1), Physics 2 (Mechanics 2), Physics 3 (Electricity) and Physics 4 (Magnetism). Of the pre-service physics teachers 28 were second year, 26 were third year, 27 were fourth year and 20 were fifth year students. The results of the data analysis indicated that the percentage of students who think that Magnetism has the most difficult subjects is the highest compared to the others. The reasons why the pre-service physics teachers experience difficulty in understanding the subjects have been grouped into four categories. (Contains 3 tables.)