Engineering design features of three types of energy sources for the Inflatable Occupant Restraint System (IORS) are reviewed. These systems use: 1. Compressed gas 2. Propellant 3. A combination of compressed gas and propellant as a power source for the inflation of the restraint cushion. An analysis is presented of each system with advantages and disadvantages related to specific design parameters. The analysis is restricted to present state-of-the-art systems. It is also based on strictly engineering design features with recognition that product cost, patent position, and other intangibles are factors in final energy source selection.The analysis indicates several important advantages of the combined propellant-air (hybrid) system over one or both of the other two. These are: 1. Ability to obtain nontoxic gases with currently developed propellants. 2. More flexibility in control of gas thermodynamics. 3. The small propellant weight requirements.