The phase and frequency of grid voltage determine the reference value of control signal of grid-connected inverter system, and ensure smooth, reliable and stable operation of the system. As the key technique obtaining the phase and frequency of grid voltage, phase-locked loop (PLL) techniques gain the attention of scholars at home and abroad. According to different properties, studies on PLL techniques are various, while literature on specialized PLL design is relatively complex. Firstly, we introduce the generation of quadrature signals of single-phase synchronous reference frame PLL and the rejection of harmonic for single-phase PLL. Because some technologies of three-phase PLL are the same with single-phase PLL, the introduction of three commonly used three-phase PLLs is given. Finally, two most critical aspects are proposed for the future development of PLL technology, including the enhancement of phase detector algorithm and the method of eliminating harmonic interference.