This article summarizes the life and career of John E. Greenleaf, PhD. It complements an interview of Dr. Greenleaf sponsored by the American Physiological Society Living History Project found on the American Physiological Society website. Dr. Greenleaf is a "thought leader" and internationally renowned physiologist, with extensive contributions in human systems-level environmental physiology. He avoided self-aggrandizement and believed that deeds rather than words define one's legacy. Viewed another way, however, Greenleaf's words define his deeds: 48% of his 185 articles are first author works, which is an unusually high proportion for a scientist of his stature. He found that writing a thorough and thoughtful discussion section often led to novel ideas that drove future research. Beyond Greenleaf's words are the many students, postdocs, and collaborators lucky enough to have worked with him and thus learn and carry on his ways of science. His core principles included the following: avoid research "fads," embrace diversity, be the first subject in your own research, adhere to rules of fiscal responsibility, and respect administrative forces-but never back down from them when you know you are right. Greenleaf's integrity ensured he was usually right. He thrived on the axiom of many successful scientists: avoid falling in love with hypotheses, so that when unexpected findings appear, they arouse curiosity instead of fear. Dr. Greenleaf's legacy will include the John and Carol Greenleaf Award for prolific environmental and exercise-related publication in the Journal of Applied Physiology.