It is argued that spiral patterns in galaxies generally are transient features which will decay away in a few dynamical times, only to be replaced by other similarly short-lived waves. The evidence for this point of view is presented and the current state of the theory is discussed. A major consequence of such behaviour is that disc stars are scattered into gradually more eccentric orbits by successive spiral waves. This has two further implications: (a) the velocity dispersion of stellar populations will increase with their ages and (b) persistent spiral structure demands a gas component in the disc that can dissipate some of the random moiton energy. That the theory offers such a matural explanation for these two well established aspects of galactic structure is cited as indirect evidence in its favour.