In an effort to characterize critical components of conceptual understanding in the domain of biology, particularly those that would be important to the design of biology instruction, I have been conducting research on individuals’ understanding of and reasoning about subcellular biological processes. In these processes, participating entities undergo sequences of dynamic changes in both internal structure and relative spatial arrangement. These dyrmmlc changes are constrained by the physical structures of the participating entities. Because subcellular biological processes occur inside cells, and thus, outside of our normal range of perception, humans have little to no direct experience with them. However, their occurrence gives rise to the observable characteristics that serve as the objects of scienUfic analyses. On the basis of such analyses, scientists construct models of subcellular processes that represent the current state of scientific understanding of the process in question. Given the model nature of the scientific understanding of subcellular biological processes, it is useful to think of developing conceptual understanding of these processes as consisting largely of constructing mental models of these processes that are consistent with the currently accepted scientific models.