With the increasing use of dynamic media in multimedia learning material, it is important to consider not only the technological but also the cognitive aspects of its application. A large amount of previous research does not provide preference to either static or dynamic media for educational purposes and a considerable number of studies found positive, negative or even no effects of dynamic media on learning performance. Consequently, it is still necessary to discern which factors contribute to the success or failure of static or dynamic media. The study presented here can be seen as another brick in the wall of understanding students' learning supported by dynamic media. In this study, aspects of cognitive load and the ability to generate mental representations for the purpose of appropriate animation design and development are considered. The learning performance of static versus dynamic media amongst a total of 129 Computer Science students, including a control group, was investigated. The results showed that learning performance using dynamic media was significantly higher than those of the static textbook lesson when the learning material had a certain level of complexity; the more complex the learning material, the larger the benefit of using animations. The results were examined for possible factors that contributed to the success or failure of dynamic media in education. In conclusion, this study has successfully confirmed the theory that dynamic media can support learning when cognitive load and learners' mental representations are taken into account during the design and development of learning material containing dynamic media. (Contains 3 figures and 1 table.)