An interpretation of the Raman spectra of monoclinic ZrO2 and monoclinic HfO2 is made by analyzing the results of the zirconia-hafnia substitution jointly with a lattice dynamical treatment of both structures. The Raman spectra of tetragonal ZrO2 and tetragonal HfO2 are also interpreted. Emphasis is put on their relations to the spectrum of the parent cubic structure and on the position of the soft mode. The band assignment proposed earlier by other researchers is critically reconsidered.