Fire on board an aircraft is a very serious hazard; all precautions must be taken to minimize the risk of a fire starting. In the event that a fire does occur, there must be adequate fire protection on the aircraft. Fire protection includes the means of both detecting and extinguishing the fire. The subject of fire protection theory is a branch of engineering in its own right. Basic fire protection theory is covered in this chapter to provide the reader with sufficient information to understand how this theory is applied through aircraft electrical systems. The chapter focuses on the equipment and systems used to detect fire and smoke together with the means of delivering the fire-extinguishing agent.