What a journey writing this text has been. The lengthy voyage started well before the idea hatched of authoring a text that contained the word "thermodynamics"! I was informed by my good friend and sometimes colleague Dr. Jose Antonio that by including that word in the title, nutritionists and exercise physiologists might avoid the subject. But almost every step of my expedition was taken on a rather solid foundation of thermodynamics and as such the topic could not possibly be omitted from the title or the text of a book about bioenergetics and energy expenditure. I am not a physicist. In fact I ?rst went to college to become a football coach. That vocational choice began to deteriorate when taking the mandatory anatomy and physiology courses required of all physical education majors. This information was exciting; my interest in physical education began to wane. During sophomore year, I answered an advertisement in the school newspaper requesting research subjects.