A brief history of the concept of negative energy waves in fluid and plasma media is given. The theory of microwave tubes is highlighted, where the small signal power theorem was first derived, which led to the interpretation of these devices in terms of the interaction of positive and negative energy waves. Three examples are described to illustrate the use of negative waves in stability problems. The first is concerned with the Kelvin-Helmholtz instability in neutral fluids. The second comes from a kinetic theory of instability in a collisionless shock wave in a magnetized plasma, and the third is concerned with the effect of a resistive wall and plasma flow on the magnetohydrodynamic stability of a magnetically confined plasma. The paper ends with some general conclusions.