W. Larry Kenney, PhD, is the Marie Underhill Noll Chair in Human Performance and a professor of physiology and kinesiology at Pennsylvania State University at University Park. He received his PhD in physiology from Penn State in 1983. Working at Noll Laboratory, Kenney is researching the effects of aging and disease states such as hypertension on the control of blood flow to human skin and has been continuously funded by NIH since 1983. He also studies the effects of heat, cold, and dehydration on various aspects of health, exercise, and athletic performance as well as the biophysics of heat exchange between humans and the environment. He is the author of more than 200 papers, books, book chapters, and other publications. Kenney was president of the American College of Sports Medicine (ACSM) from 2003 to 2004. He is a fellow of the ACSM and is active in the American Physiological Society. For his service to the university and his field, Kenney was awarded Penn State University’s Faculty Scholar Medal, the Evan G. and Helen G. Pattishall Outstanding Research Achievement Award, and the Pauline Schmitt Russell Distinguished Research Career Award. He was awarded ACSM’s New Investigator Award in 1987 and their Citation Award in 2008. Kenney has been a member of the editorial and advisory boards for several journals, including Medicine and Science in Sports and Exercise, Current Sports Medicine Reports (inaugural board member), Exercise and Sport Sciences Reviews, Journal of Applied Physiology, Human Performance, Fitness Management, and ACSM’s Health & Fitness Journal (inaugural board member). He is also an active grant reviewer for the National Institutes of Health and many other organizations. He and his wife, Patti, have three children, all of whom were Division I college athletes. Jack H. Wilmore, PhD, retired in 2003 from Texas A&M University as a distinguished professor in the department of health and kinesiology. From 1985 to 1997, Wilmore was chair of the department of kinesiology and health education and the Margie Gurley Seay Centennial Professor at the University of Texas at Austin. Before that, he served on the faculties at the University of Arizona, the University of California, and Ithaca College. Wilmore earned his PhD in physical education from the University of Oregon in 1966. Wilmore published 53 chapters, more than 320 peer-reviewed research papers, and 15 books on exercise physiology. He was one of five principal investigators for the HERITAGE Family Study, a large multicenter clinical trial investigating the possible genetic basis for the variability in the responses of physiological measures and risk factors for cardiovascular disease and type 2 diabetes to endurance exercise training. Wilmore’s research interests included determining the role of exercise in the prevention and control of both obesity and coronary heart disease, determining the mechanisms accounting for alterations in physiological function with training and detraining, and factors limiting the performance of elite athletes. A former president of the American College of Sports Medicine (ACSM), Wilmore received ACSM’s Honor Award in 2006. In addition to serving as chair for many ACSM organizational committees, Wilmore was on the United States Olympic Committee’s Sports Medicine Council and chaired their Research Committee. He was a member of the American Physiological Society and a fellow and former president of the American Academy of Kinesiology and Physical Education. Wilmore consulted for several professional sport teams, the California Highway Patrol, the President’s Council on Physical Fitness and Sports, NASA, and the U.S. Air Force. He also served on editorial boards of several journals. Wilmore passed away during the preparation of the sixth edition of this text. David L. Costill, PhD, is the Emeritus John and Janice Fisher Chair in Exercise Science at Ball State University in Muncie, Indiana. He established the Ball State University Human Performance Laboratory in 1966 and served as its director for more than 32 years. Costill has written and coauthored more than 430 publications over the course of his career, including six books and articles in both peer-reviewed and lay publications. He was the original editor in chief of the International Journal of Sports Medicine for 12 years. Between 1971 and 1998, he averaged 25 U.S. and international lecture trips each year. He was president of the American College of Sports Medicine (ACSM) from 1976 to 1977, a member of its board of trustees for 12 years, and a recipient of ACSM’s Citation Award and Honor Award. He has received numerous other honors, including an honorary doctoral degree from the Stockholm School of Physical Education, the Professional Achievement Award from Ohio State University, the President’s Medal of Distinction at Ball State University, and the Distinguished Alumni Award from Cuyahoga Falls Public Schools. Many of his former students are now leaders in the fields of exercise physiology, medicine, and science. Costill received his PhD in physical education and physiology from Ohio State University in 1965. He and his wife of 58 years, Judy, have two daughters. Now retired, Dr. Costill is a private pilot, auto and experimental airplane builder, competitive masters swimmer, and former marathon runner.