Ever since we demonstrated the first nanogenerators using piezoelectric nanowires for converting mechanical energy into electricity (Wang & Song, Science, 312, 242-246 (2006)), a great interest has been excited worldwide for developing various approaches for energy harvesting. A key idea presented in the 2006 paper is the self-powered nanotechnology, aiming at powering nanodevices/nanosystems using the energy harvested from the environment in which the systems are suppose to operate. We have published a series of papers during a period of 6 years covering the basic theory, mechanism, engineering scale up and potential applications of nanogenerators (see Appendix). To provide a comprehensive and coherent review about the development of nanogenerators, I have organized this book based on our published papers. The readers are encouraged to referring to the original papers for details and citations.