This book aims to prove that the so-called "energy crisis" is really an entropy crisis. Since energy is conserved, it is clear that a different concept is necessary to discuss meaningfully the problems posed by energy supplies and environmental protection. This book makes this concept, entropy, accessible to a broad, nonspecialized audience.Examples taken from daily experiences are used to introduce the concept of entropy in an intuitive manner, before it is defined in a more formal way. It is shown that the entropy increase due to irreversible transformations (or "unrecoverable" energy) simultaneously determines the level of fresh energy supplies of our society and the damage that it causes to the environment. Minimizing the rate of entropy increase with advanced technologies and society organizations, and keeping it in check with appropriate energy sources, is the key to a sustainable development.