This chapter provides a comprehensive overview of the polymerase chain reaction (PCR), a cornerstone technology in molecular biology. It begins with a brief historical introduction and a detailed explanation of the fundamental principles and system design of PCR, including reaction components, basic thermal cycling steps, and challenges such as non-specific amplification. Various product detection methods, particularly agarose and polyacrylamide gel electrophoresis, are discussed. The chapter extensively explores advanced PCR variants and their specific mechanisms. Key techniques covered include real-time fluorescence quantitative PCR, reverse transcription PCR (RT-PCR), multiplex PCR (along with advanced multiplex strategies such as MLPA and MeltArray), as well as specialized formats like touchdown PCR, nested PCR, inverse PCR, asymmetric PCR, in situ PCR, immuno-PCR, and digital PCR (with emphasis on droplet digital PCR). Additionally, the chapter addresses critical issues of PCR contamination and control measures. It concludes with an overview of the broad applications and future perspectives of PCR technology, highlighting its continuous evolution and enduring significance in genetic analysis and diagnostics.