Living standards reflect strong correlations between a country's gross domestic product and energy consumption per capita, making it quite natural for all countries to try to increase energy consumption. The net effect is a significant increase of energy demands and environmental impacts. Modern power systems are made up of distinct sections: generation, transmission, distribution, and end-users. The electric power system is a complex technical system whose main function is to deliver electricity between generation, consumption, and storage. Special attention is given to power distribution and industrial electricity systems. The overall goal of this book is to provide engineers, students, and interested readers with the essential knowledge of building and industrial energy systems; main technologies; and how they work, operate, and are evaluated and selected for specific applications. A basic review of the concepts of work, energy, power, heat, energy transfer, and conversion is included in this chapter. These concepts are critical for the understanding of many other topics discussed in the book, ranging from renewable energy, thermal conversion, energy storage, distributed generation, lighting or heat, air-conditioning, or electric power systems. A brief discussion of units, unit systems, dimensional analysis, unit conversion, and common energy units is also included in this chapter. Engineers, technicians, and design professionals, as well as people in charge of building operation, make judgments and calculations about power, lighting, heating and cooling requirements, space, location of components and equipment, safety, and costs, by using concepts, theories, units, and energy conversion discussed in this chapter.