The inert gas system, or the system for inertiating the closed atmosphere of cargo tanks, is the leading factor in protection against explosions of ships carrying flammable cargo. The function of the inert gas is based on the simple isolation of oxygen, as a factor supporting combustion, due to the difference in densities and pressure of gas supplied to the enclosure. The paper discusses the basic properties of gases encountered on board as mixing products from the surrounding atmosphere and gases caused by harmful evaporation of cargo and the mode of action of inert gas and the formulation of explosive mixtures in cargo tanks. The principles of operation of flue gas purification systems, topping-up generators and nitrogen generators and the role and routine control of certain safety mechanisms of inert gas systems are presented, as well as the necessary precautions in daily work in hazardous environments.