The explosion of knowledge in all of the biological sciences, and specifically in physiology, has created a growing problem for educators. There is more to know than students can possibly learn. Thus, difficult choices have to be made about what we expect students to master. One approach to making the needed decisions is to consider those "core principles" that provide the thinking tools for understanding all biological phenomena. We identified a list of "core principles" that appear to apply to all aspects of physiology and unpacked them into their constituent component ideas. While such a list does not define the content for a physiology course, it does provide a guideline for selecting the topics on which to focus student attention. This list of "core principles" also offers a starting point for developing an assessment instrument to be used in determining if students have mastered the important unifying ideas of physiology. (Contains 3 tables.)