It has been theoretically predicted and experimentally verified that spin-wave instabilities can be excited in a ferromagnetic material when a microwave magnetic field is applied parallel to the d.c. field. The paper presents a summary of a plane-wave theory concerning this phenomenon and the results of the experimental verification of it. The utilization of the phenomenon as a new investigation tool for the study of relaxation mechanisms in ferromagnetic materials is discussed and the possibilities of application in the field of microwave devices are considered.