This chapter discusses the selection of piezoelectric materials and circuit design in vibrational energy harvesting. The performance of energy-harvesting unimorph devices that captured frequencies of 60 Hz using PZT-based and BT-based ceramics was evaluated. Output voltages and power depend on the amplitude and frequency of oscillations and the load resistance. PZT-based ceramics are generally superior for piezoelectric energy-harvesting applications. The figures of merit of the materials are discussed to provide guidelines for material selection. The piezoelectric voltage coefficient, $ g_{31} $, is considered a good parameter to predict maximum voltages. On the other hand, $ d_{31}g_{31}/\tan\delta $, $ k_{31}^{2}Q_{m} $, and $ d_{31}g_{31} $ are close to the behavior of maximum power. The combination of the piezoelectric unimorph and power management circuit produced constant voltage output, which could be used as a power source. Energy harvesting (EH) is the process of capturing small amounts of energy from external sources and storing them for later use. EH devices can be used for self-sufficient energy supply systems. The main application of EH is for independent sensor networks. The available energy from the environment includes solar power, thermal energy, wind energy, salinity gradients, and kinetic energy. Mechanical vibration is the most attractive alternative for EH. Vibration-electrical energy harvesting using piezoelectric effects has been explored for use in sensor network modules. To harvest energy from the environment, it is necessary to capture vibrations with frequencies less than 200 Hz, as these frequencies are dominant in normal life and in vehicles. The power management circuit plays an important role in the system. The block diagram of the power management circuit is shown. Since the voltage and current of the electricity generated by the piezoelectric energy harvester are alternating, a diode rectifier is required to produce direct current (DC) power supply. The electricity from the piezoelectric power generator has large amplitude and frequency fluctuations, so a regulation circuit is required. The output voltage from the piezoelectric generator, the output voltage after rectification, and the controlled voltage output are shown. Alternating voltage with a waveform of almost positive-negative symmetry is generated by the piezoelectric generator. By full-wave rectifying circuit, the generated voltage is converted to one of constant polarity (positive or negative). Smoothing capacitor or filter is required to produce steady direct voltage. The regulated circuit including capacitor and DC-DC converter produced constant voltage output. To enhance the performance of the energy-harvesting circuit, nonlinear processing techniques such as "synchronized switch harvesting on inductor" (SSHI) or active full-wave rectifier by using CMOS technology, and power conditioning circuit with maximum power point tracking (This chapter discusses the selection of piezoelectric materials and circuit design in vibrational energy harvesting. The performance of energy-harvesting unimorph devices that captured frequencies of 60 Hz using PZT-based and BT-based ceramics was evaluated. Output voltages and power depend on the amplitude and frequency of oscillations and the load resistance. PZT-based ceramics are generally superior for piezoelectric energy-harvesting applications. The figures of merit of the materials are discussed to provide guidelines for material selection. The piezoelectric voltage coefficient, $ g_{31} $, is considered a good parameter to predict maximum voltages. On the other hand, $ d_{31}g_{31}/\tan\delta $, $ k_{31}^{2}Q_{m} $, and $ d_{31}g_{31} $ are close to the behavior of maximum power. The combination of the piezoelectric unimorph and power management circuit produced constant voltage output, which could be used as a power source. Energy harvesting (EH) is the process of capturing small amounts of energy from external sources and storing them for later use. EH devices can be used for self-sufficient energy supply systems. The main application of EH is for independent sensor networks. The available energy from the environment includes solar power, thermal energy, wind energy, salinity gradients, and kinetic energy. Mechanical vibration is the most attractive alternative for EH. Vibration-electrical energy harvesting using piezoelectric effects has been explored for use in sensor network modules. To harvest energy from the environment, it is necessary to capture vibrations with frequencies less than 200 Hz, as these frequencies are dominant in normal life and in vehicles. The power management circuit plays an important role in the system. The block diagram of the power management circuit is shown. Since the voltage and current of the electricity generated by the piezoelectric energy harvester are alternating, a diode rectifier is required to produce direct current (DC) power supply. The electricity from the piezoelectric power generator has large amplitude and frequency fluctuations, so a regulation circuit is required. The output voltage from the piezoelectric generator, the output voltage after rectification, and the controlled voltage output are shown. Alternating voltage with a waveform of almost positive-negative symmetry is generated by the piezoelectric generator. By full-wave rectifying circuit, the generated voltage is converted to one of constant polarity (positive or negative). Smoothing capacitor or filter is required to produce steady direct voltage. The regulated circuit including capacitor and DC-DC converter produced constant voltage output. To enhance the performance of the energy-harvesting circuit, nonlinear processing techniques such as "synchronized switch harvesting on inductor" (SSHI) or active full-wave rectifier by using CMOS technology, and power conditioning circuit with maximum power point tracking (