The existing buildings in several countries are characterized by numerous examples of RC frames with masonry infills. Various studies showed how the presence of masonry infills may alter the overall behaviour of structures when subjected to seismic action. The presence of the masonry infills can produce positive or negative effects. In the first case it can increase the strength and the dissipative capacity of the structure, in the second case it can produce unexpected distributions of forces and consequent local phenomena of collapse. The objective of this paper is the assessment of the seismic vulnerability of existing infilled RC frames. In order to evaluate the behaviour of these structures, it is fundamental to define an appropriate analytical model which could take into account the presence of the infills. Several models have been proposed in literature for the masonry infills. In this study the equivalent strut model is considered. The study starts from the model for monotonic loading proposed by Al-Chaar. This model is extended to the case of cyclic loading by calibrating the degradation of strength and stiffness, the residual strength and the loading and unloading branches. The model is then applied for investigating the seismic response of infilled RC frames: pushover and nonlinear dynamic analyses are carried out for obtaining the response in terms of base shear-top displacement and for estimating the configuration of collapse. In particular the lateral displacement profile at collapse and the plastic hinge distribution are examined. A proposal for the extension of the Displacement-Based Assessment to infilled frames is then presented and verified. In this assessment procedure it is implemented a simplified method proposed by the authors for the prediction of the displacement profile of infilled frames at the yield and collapse conditions. This method is applied together with specific ductility damping relationships derived by the authors in previous studies. The proposed assessment procedure is then applied to the considered cases of study and its effectiveness is verified through comparisons with the results of the nonlinear dynamic analyses.