The effect of masonry infills on seismic behaviour of reinforced concrete frames is described. In the first part of the thesis, the theoretical background and procedure for seismic analysis of such structures according to Eurocode 8 are presented. The effect of masonry infill was demonstrated by means of an example of four-storey reinforced-concrete building, including the consideration of the effect of soft storey mechanism. All linear elastic analyses were performed with ETABS software. The main objective of the study was to assess the impact of the modelling uncertainties on the global and local engineering demand parameters. In addition to the classical macro model of infill, which is based on single-strut approach, the infill panel was modelled by the two-strut or the three-strut approach as well as by the shell elements taking into account the nonlinear contact between the masonry infill and the adjacent frame. It was found that the macro models based on multiple struts can simulate local effects on columns with slightly greater accuracy as those obtained on the basis of single-strut model. However, it is shown that the local effects on columns due to infills can be simulated also by the simple single-strut model if a certain proportion of the force in the strut is considered for the evaluation of the internal forces in the columns.