Abstract: A method and device for monitoring large structures so as to detect defects, e.g. cracks. An electric current is supplied to a steel structure which is equipped with contact points between which are measured voltage drops caused by the impressed current. A relatively large number of fixed contact points are used all over the area which is to be monitored. The voltage drops are measured between selected pairs of contact points and these voltage drops are compared with corresponding voltage drops that have been measured previously in the same manner when the structure was in an initial condition, preferably without any defects. The monitoring can this be performed by means of robust and simple devices which are relatively impervious to rough environments.