<P>This book describes the fundamentals and principles in the field of energy harvesting. It provides the necessary theory and information to develop energy harvesting power supplies. The overall system design is explained and quantitative assumptions on environmental energy are given. Each one of the blocks that compose an energy harvesting power supply is analyzed and trade-offs related to the design of the required buildings blocks are discussed. Different energy transducer technologies like piezo-electric, electro-dynamic, solar-cells and thermo-electric generators are covered. </P>