This article analyzes the current research status of phase-locked loops (PLLs) from multiple aspects. The working principle and components of PLL are discussed in detail, including the feedback controlling mechanism, clock skew generation and elimination, and frequency multiplication. The main components of PLL, including phase detector, low-pass filter, and voltage-controlled oscillator, are also explained in the following parts. The article further explores the applications of PLL, such as frequency synthesizers and clock and data recovery, and the challenges faced in designing PLLs. These challenges include the need for new architectures and advanced loop filters and improved phase detectors. Finally, the prospects and potential developments in PLL technology will be given. The paper will first present a comprehensive review of recent advances in PLL research, focusing on both the theoretical and practical aspects of PLL design and application. Then highlight the innovative approaches proposed by researchers to address the challenges associated with PLLs, including new architectures, advanced loop filters, and improved phase detectors.