Superfluid helium is rich in the number of wave propagation modes which occur because it behaves as though it is composed of two fluids one of which is superfluid and the other a normal fluid. Four recognized modes are named first, second, third and fourth sound, of which all but the first occur only in quantum fluids. First sound is an ordinary compressional wave. Second sound is a propagating entropy wave. Third sound is a thickness wave which propagates in helium films and has been observed in films as thin as two atomic layers. Fourth sound is a compressional wave which propagates when the normal fluid component is locked as in the small pores of a rigid porous medium. These sound waves provide a powerful means of investigating the superfluid properties of helium and examples of such use is described. [Work supported by ONH and NSF.]