One of the sub-systems of air bag passive restraint systems presently under examination for automotive application is the energy source sub-system. The function of the energy source is to provide, release, and control a volume of gas at a rapid rate to inflate the air bag. Three basic types of energy sources will be described, these being stored gas, generated gas, and hybrid sources. Both liquid cooled and gas cooled hybrid sources are mentioned. The automotive requirements for application of energy sources are discussed in terms of performance objectives for passive restraints, particularly for front seat passenger air bags. The requirements include minimum package size and weight, ability to perform in hot and cold as well as normal environments, freedom from degradation during the service life, high reliability, controlled toxicity and heat, and ability for safe disposal during vehicle recycling. Two new types of inflation systems which are to improve versatility are mentioned. These systems are to provide either secondary inflations or multiple deployment speeds depending on their design.