Alignment to buried marks is carried out by using electromagnetic radiation to induce waves in the layers covering the buried layer. The acoustic or thermal waves cause reflectivity changes and displacements in the surface whose position and/or time dependence reveals the true position of the buried alignment mark. The buried alignment mark may be revealed by mapping the thickness of covering layers in its vicinity, e.g. by measuring the time dependence of the decay of a standing wave induced in the covering layers or by measuring the delay time of echoes of a travelling wave created at interfaces between different ones of the covering layers. Alternatively, a travelling wave can be created over the whole area of the mark so that echoes off the top and bottom of the buried mark carry positive and negative images of the mark; these cause reflectivity differences and displacements when they reach the surface which can be aligned to.