Service loads, environmental and accidental actions may cause damage to constructions. Regular inspection and condition assessment of engineering structures are necessary so that the early detection of any defect can be made and the structure's remaining safety and reliability can be determined. When the structural damage is small or it is in the interior of the system, its detection cannot be carried out visually. A useful more elaborate non-destructive evaluation tool is vibration monitoring. It relies on the fact that the occurrence of damage or loss of integrity in a structural system leads to changes in the dynamic properties of the structure. In this paper, different techniques will be presented and compared to derive from experimentally determined modal characteristics of a reinforced concrete beam its dynamic bending stiffness. The degradation of stiffness, due to the cracking of the reinforced concrete, gives information on the position and severity of the damage that has occurred. (C) 2000 Elsevier Science Ltd. All rights reserved.