Detailed investigations of lateral inhomogeneities of the upper mantle properties are of major significance in the Upper Mantle Project. The paper gives a review of the present status of such research as far as seismological methods are concerned. These include studies both of stress systems and strain release (earthquakes) and of all kinds of seismic waves (body waves, guided waves, surface waves). There are certain methodical difficulties both in general (especially concerning the scale of any inhomogeneities which can be discovered) and in relation to special methods. Although seismology provides the most unambiguous data, there is great need for close coordination of results from other geophysical disciplines. Several ideas and suggestions in the paper have not yet left the level of hypotheses, but if they could serve as stimulus for further research, the purpose of the paper has been more than fulfilled.