The term "conceptual understanding" has been used rather loosely over the years in educational practice, with a tendency to focus on a few aspects of an extremely complex phenomenon. In this first article of a two-part miniseries on conceptual understanding, we describe the nature of expert (versus novice) knowledge and show how the conceptual understanding of experts is multifaceted in nature requiring competence in a wide range of cognitive skills. We then discuss five such facets of conceptual understanding that require competence in the cognitive skills of memorization, integration, transfer, analogical reasoning, and system thinking. We also argue for the importance of explicitly teaching and assessing such facets of understanding as part of all molecular life science curricula so as to better prepare our students to become experts in the field. Examples of the assessment tasks that can be used to promote the development of multifaceted conceptual understanding in students are presented in Part 2 of this series. (Contains 1 table.)