ABSTRACT
The source, amount, and quality of available energy determines the kinds and numbers of organisms in nature and, ultimately, the number and lifestyle of humans. Recognition that energy is not only the common denominator of humans and environment but also the ultimate limiting factor for development is of key importance if we are to maintain human quality of life in the future. To document this overall statement, we need to review how energy is measured, how much is needed for basic life-supporting activities, the thermodynamic laws that determine how energy is used, and the importance in considering quality as well as quantity.
