The significant growth of portable electronics has been the driving force for the development of a power source to generate energy from the ambient environment. Currently, most of the portable electronics depend on a battery system for its power source. However, the battery has a limited lifetime, requires charging and maintenance. Thus, for the development of next-generation electronics devices, it is important to make these devices self-powered. The crucial component of such a system is a power source that can harness energy from the ambient environment. Among the various energy harvesting technologies, a mechanical energy harvester has evolved as a promising alternative owing to its ability to harvest energy from various random mechanical vibrations present in the ambient environment. These vibrations are present everywhere and at any time. Mechanical energy can be harnessed utilizing various mechanisms such as electrostatic, triboelectric and piezoelectric. Although the piezoelectric effect has been known for 100 years, it has recently gained significant attention with the advent of nanotechnology which catapulted its energy harvesting performance by several orders of magnitude. This chapter will highlight both the fundamentals and the recent development of piezoelectric energy harvesters.