Electronic textbooks, or e-texts, will have an increasingly important role in college science courses within the next few years due to the rising costs of traditional texts and the increasing availability of software allowing instructors to create their own e-text. However, few guidelines exist in the literature to aid instructors in the development and design specifically of e-texts using sound learning theories; this is especially true for undergraduate physiology e-texts. In this article, we describe why constructivism is a very important educational theory for e-text design and how it may be applied in e-text development by instructors. We also provide examples of two undergraduate physiology e-texts that were designed in accordance with this educational theory but for learners of quite different backgrounds and prior knowledge levels.