The dynamic analysis of structures with piezoelectric sensors and actuators is used in this paper to establish a method for crack detection in aerospace structures. Piezoelectric strips used as sensors and actuators are bonded on both sides of a thin structure which executes flexural oscillations. The differential voltage outputs of the piezoelectric sensors are used to detect the presence of cracks in the structure. The structural analysis uses a finite element formulation for the piezoelectric strips coupled with the structure and a nonlinear model for the cracks. This paper presents first the results of the dynamic analysis in the frequency domain of healthy and cracked plates undergoing forced flexural vibrations generated by a pair of piezoelectric actuators submitted to an oscillatory voltage excitation. The peaks in the differential voltage output obtained in the case of a cracked plate at several frequencies during the frequency sweep were found to be indicative measures for the presence of a crack in the structure. The results of the dynamic analysis in the time domain have also shown that this method has a good sensitivity in detecting cracks in the structures.