Prechamber is an advanced ignition technology used in spark ignition engine combustion. The prechamber ignition system allows the engine to operate for a lean air–fuel mixture by enhancing turbulence by an order of magnitude. The objective of using this technology is to achieve low emission and high engine brake thermal efficiency. The two types of prechambers are active and passive. An active prechamber can achieve ultra-lean combustion of fuels with high brake thermal efficiency. A passive prechamber operates at a lean limit slightly better than a conventional spark ignition engine. However, the advantages of the passive prechamber are high knock resistance, short combustion duration, and combustion stability. The objectives of prechamber design are to achieve fast flame propagation in the prechamber, proper scavenging, and high turbulence in the main chamber. Also, the prechamber is designed so that minimum modifications in the engine are made, and it can be easily replaced with a spark plug. The different design parameters of the prechamber are prechamber volume, the shape of the prechamber, nozzle diameter, the number of orifices, and the diameter of the orifice. The current chapter gives an introduction and history of prechamber concept development. This chapter reviews the state of the art of designing different parameters of the prechamber. The physical phenomenon occurring inside the prechamber and its importance in efficient design have been discussed. The effect of different prechamber parameters on physical phenomena in the prechamber, combustion, emissions, and performance of an engine, such as flame front propagation, charge motion in the prechamber, jet ejection, and scavenging processes, in-cylinder pressure, heat release rate are discussed. This chapter also discusses constant-volume combustion chambers, rapid compression machines (RCM), and engine studies conducted for the design optimization of prechambers. A summary of normalized optimum prechamber parameters based on the literature survey range is tabulated, which can be used by the readers as a reference for their design. Overall, this chapter gives an overview of the design of the prechamber and suggests the identified research gaps.