The fourth state of matter, the "plasma state," although relatively rarely found at the surface of the earth, is the most commonly found state of matter when the known universe is considered. The plasma state, consisting of a neutral collection of electrons and positive ions may have a very wide range of densities. The production and properties of this state are described. Plasmas are of particular interest now because of the possibility of initiating neuclear fusion in them but they also appear in phenomena ranging from those found in a neon sign, a lightning stroke, the ionosphere about the earth, shock waves, and the compressed layer of hot gas around an astronaut's capsule as the comes back through the earth's atmosphere.