This book is a first effort to fill the need for a comprehensive text on energy. The importance of energy and its effects on everyday life is undisputable. Consequently, many institutions today offer either minor or major degree programs on energy. This undergraduate textbook is prepared for students with diverse backgrounds who are interested to know more on energy and pursue a degree on energy. Although, it is naturally connected to thermodynamics through the energy balance, the new textbook covers many aspects of energy in systems with rate and transport processes.