In this paper the sensitivity of the modal damping and natural frequencies of adhesively bonded and composite beams to small pre-deformations is investigated. Originally, a set of experiments was conducted on adhesively bonded beams in order to assess the extent to which damping could be influenced by the joint's location. As a part of the study, the experiments were repeated a number of times, and it was observed that the data exhibited a large amount of scatter. After further investigation, it was found that the scatter in the damping data could be correlated with small, initially undetected pre-deformations of the beams, and that the pre-deformation also caused significant changes in the beams' natural frequencies. In order to study this phenomenon further, a finite element model of the specimen was developed that could be used to calculate its mode shapes, natural frequencies and modal damping. The finite element model exhibited behavior that was consistent with the experiments. The calculations also indicated that if the frequencies were plotted as a function of the amplitude of the pre-deformation, then some pairs of frequencies crossed while others veered away from each other, and that the crossings and veerings could be closely related to mode shape changes and damping variations. In addition, the finite element program was used to simulate the behavior of composite sandwich beams and it was predicted that they would exhibit analogous behavior. Lastly, sets of experiments were conducted on both adhesively bonded and composite beams in which the amplitudes of the pre-deformation, the natural frequencies and the modal damping were measured and compared with predictions from the finite element model. The correlation between the predictions and the experimental data was good and the amount of scatter was reduced to acceptable levels once pre-deformation was taken into account.