This paper gives a detailed report of the evolution and potential applications of piezoelectric nanogenerators (PENGs). Various configurations, together with the operating principles and techniques used for the fabrication of PENGs, are discussed. A brief overview of the wide range of materials used for the design of PENGs that exhibit piezoelectric properties is also reported here. The optimization parameters to be considered while designing a PENG are also presented. PENGs are used in several application areas, which are collectively summarized.