The ability to deliver sustainable electric power to micro-electro-mechanical systems (MEMS) or a wireless system network by energy harvesting is attractive not only because of the cost of batteries but also because it removes the additional time and cost that is necessary to replace and maintain the batteries, including the installation of complex wired systems. In particular, this feature is relevant to the installation of sensor nodes in areas that are dangerous or difficult to reach, e.g., safety-monitoring devices, structure-embedded microsensors, and medical implants. Furthermore, there are environmental benefits associated with limiting or eliminating the disposal of batteries. Thus, EH devices provide a “battery-less” solution by scavenging energy from ambient energy sources, such as light, heat, water, and mechanical vibrations, and converting it into useable electrical power. There are also advances in MEMS, microdevices, and nanodevices: microprocessor technology that has increased power efficiency and reduced power consumption. Energy-storage solutions are also improving, such as the development of super-capacitors and even structural power that will ultimately lead to successful EH products and systems. This chapter provides a list excellent publication reviews and papers in the areas of piezoelectric energy harvesting. It reviews EH technologies, associated “piezoelectric” materials, and the fundamental behavior of piezoelectricity for applications in sensors and EH technologies. In fact, many devices and applications are constantly evolving depending on smart-materials technology, such as scanning probe microscopes (SPMs) and cigarette lighters. Today, vibration-based EH via piezoelectric materials has become one of the most prominent ways to provide limited energy for self-powered wireless sensors and low-power electronics. An insight that involves the mathematical modeling of constitutive equations, lumped parameter model, mechanisms of piezoelectric energy conversion, and operating principle of a piezoelectric EH system is provided. It also focuses on the dielectric, piezoelectric, mechanical, and pyroelectric properties of piezoelectric and pyroelectric materials open to use from a single crystal, such as PMN-PT, ceramics (PZT), and polymers (PVDF). Recent important literature is also reviewed.