The aim of this study is to identify the opinions of pre-service physics teachers about the difficulties in introductory quantum physics topics. In this study conducted with twenty-five pre-service physics teachers, the case study method was used. The participants were interviewed about introductory quantum physics topics. The interviews were coded using content analysis and were divided into different categories. In this way, we tried to reveal the opinions of pre-service physics teachers about the difficulties of these topics. In conclusion, it can be said that the participants had the most difficulty in transition from classical physics to quantum physics. This is especially evident in quantum physics topics that require a different understanding such as black body radiation, wave-particle structure of light and the uncertainty principle. It was also found that the participants had no difficulties in terms of mathematical calculations, and that approaches based on visualizing the topic were received positively.