Abstract: A laser beam from a source 20 is used to provide a reference in space adjacent a portion of a structure movements of which are monitored. The beam is received by a receiver 30. A number of targets 300 may be mounted to move with parts of the structure extending alongside the beam path. Target constructions (FIGS. 6-8) are described which allow for normal passage of the beam past the target but which enable the position of the beam relative to a given target to be measured by intercepting the beam. In a variation (FIG. 13) for civil engineering use, a single target is selectively mountable to one of a number of supports. Another variation (FIG. 11) uses graduated mesh targets on which the beam impinges on each mesh but is also transmitted through the mesh. Dynamic movement of a part of structure such as a bridge is measured with a single target attached to the part in question and comprising a barred reflective assembly (FIG. 3), a fibre optic array (FIG. 4) or a position sensing photo-diode (FIG. 5).