In this review article, which includes some new materials, the concepts of availability and exergy are discussed with easy adoption in mind to the efficiency analysis of process and energy facilities. Also discussed are the clarification of the terms; energy, available energy, availability, exergy and anergy together with the exergetic balance. An illustrative application to the medium-Btu coal gasification process is presented by reproducing the work of Gaggoli and Petit for the tutorial purpose. In the application the physical significance of the exergy losses, magnitude and the distribution of these losses in subsystems are interpreted. In Part II, which will appear in the next issue, the contrastive concept of lost energy analysis will be discussed based on the irreversibilities in the actual processes.