Abstract: Displacement or strain is measured by applying to a body to be monitored at least one grid line (two or more where strain measurements are required) and scanning a radiation beam such as a laser beam across or partly across the grid line(s) to produce a pulsed output from which the mean position(s) of the grid line(s) can be ascertained with reference to a datum point on the beam scanning path. Displacement of each grid line relative to the datum point can be determined. The beam scan may be adjustable to produce a null condition in which the mean position of the grid line(s) coincides with the mid-point of the beam scan. Any deviation from the null condition then indicates displacement of the body or strain thereof.