Abstract: A method for measuring deformations of test samples in testing machines is carried out in a device including a light source, a detector, and signal processing circuitry. The deformations are sensed in that a light beam (2) from the light source (1) is reflected by at least one mirror (6, 6', 6a, 6b, 11a, 11b) arranged at an appropriate location, for example, directly on the test sample (4) so that test sample movements are imparted directly or indirectly to the reflecting mirror or mirrors, whereby the reflected light beam impinges upon a position detector (7) and the position of the impingement or rather its movement is determined and evaluated in an electronic evaluating circuit (8). This movement is a direct measure of the deformation. The device includes components for carrying out these method steps, whereby it is possible to measure static and/or dynamic deformations caused by tensile and compressive forces, as well as deformations resulting from twisting torques and bending moments.