Familiar as we are with three states of matter in our everyday surroundings - solid, liquid and gas - we may be surprised to learn that most of the known Universe is in a fourth state. This fourth state of matter known to physicists as plasma, is a fluid of electrically charged particles, often at very high temperatures. It is the stuff of which stars are made; at one time in the early Universe it was the state in which all matter was found. It occurs on Earth in lightning discharges and, more prosaically, in fluorescent light tubes. This book allows a glimpse through the plasma in our Universe. In The Fourth State of Matter a husband-and-wife team explain to the lay reader in simple English the nature and properties of plasma, its formation and what it's used for. Most importantly for future generations, research in plasma physics may help to control thermonuclear reactions and provide a virtually inexhaustible supply of energy.