Dynamic concepts are difficult to explain in traditional media such as still slides. Animations seem to offer the advantage of delivering better representations of these concepts. Compared with static images and text, animations can present procedural information (e.g. biochemical reaction steps, physiological activities) more explicitly as they show the steps in an orderly manner. Quite a few empirical studies showed promising results animations have on learning (e.g. Trevisan, Oki and Senger, 2009; Hays, 1996). There are, however, also limitations. Designing and developing quality animations for teaching and learning can be challenging sometimes (Morrison, Tversky and Betrancourt, 2000). Kesner and Linzey (2005) even found no improvement on students' learning in using animations in their study. It thus occurs to the researchers that there are factors that govern successful use of animation in teaching and learning. Our study explored such factors in the context of physiology teaching. 913 students in twelve different classes (collected in two stages, four years in total) in the same physiology course learned complicated microscopic mechanisms with assistance from animations provided as supplementary materials primarily for self-study. Surveys and group interviews were conducted that provided both qualitative and quantitative feedback. Results were mostly positive--animations surely explain contents more explicitly to students (especially for the explanation of dynamic and complicated biological processes), make students more interested in the subjects taught; and there is a greater demand for similar learning tools from the students. It is strongly believed that animations are good supplementary learning materials for students particularly for learning complicated concepts. Important success factors we found included the detailed explanation of content, a good balance between clear presentation and beautiful interface, the speed of running/ loading of the animations, and the provision of more references, etc. (Contains 4 tables and 2 figures.)