A method of measuring biaxial stress in an object of a ferromagnetic material in which material in a region (10) in the vicinity of a surface of the object is subjected to a conditioning method by application of a conditioning magnetic field that is at least initially at a high field strength. Values of biaxial stress within the said region are measured with an electromagnetic measuring probe (14) in at least two different orientations, the electromagnetic measuring probe (14) using an alternating measuring magnetic field that is at a field strength well below saturation. The conditioning may subject the region (10) to a low frequency alternating magnetic field (38, 58) initially at a high field strength, and gradually reducing the strength to zero over a decay time period at least equal to the time for many cycles of the low frequency magnetic field. Conditioning the material enables the stress to then be measured more accurately, and enables ambiguities in biaxial stress to be resolved.