The hydraulic impact mechanism serves as the core component of hydraulic rock drills and hydraulic breakers. It is characterized by high efficiency and energy savings, and is widely applied in fields such as mining and civil engineering. This chapter outlines the development history and classification of hydraulic impact mechanisms, and analyzes the working principles of the two main types: rear-control and dual-control. In addition, it highlights the unique features that distinguish hydraulic impact mechanisms from conventional hydraulic machinery, such as the dependence of working oil pressure on inertial forces and the drastic fluctuations in pressure and flow rate. Finally, the chapter analyzes the current major research approaches and outlines their respective characteristics.