Abstract A method is presented in this paper which uses the modal parameters of the lower modes for the non-destructive detection and sizing of cracks in beams. Using a finite element model of the structure to calculate the dynamic behaviour analytically, it is possible to formulate the inverse problem in optimization terms and then to utilize a solution procedure employing genetic algorithms. The damage assessment technique has been applied both to simulated and to experimental data related to cantilevered steel beams, each one with a different damage scenario; i.e., the position and depth of the cracks. It is demonstrated that this method can detect the presence of damage and can estimate both the crack positions and sizes with satisfactory precision. The problems related to the tuning of the genetic search and to the virgin state calibration of the model are also discussed.