The concept of diffusion may seem easy enough for many teachers to understand, but when and what do students really understand about diffusion and osmosis? In order to answer these two questions, 116 students (46.6% in the tenth grade, 30.2% in the eleventh grade, and 23.2% in the twelfth grade) enrolled in biology classes were administered tests to assess for their level of cognitive development and their understandings of diffusion and osmosis. Results revealed that while there were significant differences in level of cognitive development, significant differences between grade levels and gender did not exist. The researchers asked teachers to not assume that a concept can be easily understood by students simply because it is basic knowledge. Instead they should be aware of the fact that students must have a working knowledge of other concepts before they can truly understand more abstract concepts. (ZWH)