Infilled frames are composite structures that consist of a frame (reinforced concrete or steel) and infill for which is the clay or even concrete block commonly used in Croatia. A similar structural system called confined masonry consists of the masonry walls with vertical and horizontal confining elements. In infilled frame systems the infill is built after frame is hardened. As opposed to the infilled frame system, in the case of confined masonry, the masonry walls are built first and then confining elements around them. Infill significantly alters the structural response under earthquake load and its influence must be taken into account. Current regulations do not provide explanations about it and how it should be calculated. With a very detailed explanation of frames and other common reinforced concrete structures and modern regulations available to designers, the lack of explanation on the infilled frames can be attributed to insufficient understanding of their influence as well as to a number of uncertainties involved in infill modeling. The experiences from past earthquakes have shown the favorable and unfavorable behavior of infilled frame structures. However, modern regulations are mainly oriented to the adverse effects, which results with oversized and uneconomical design. The interaction between unreinforced infill and the frame can lead to unusual behavior of the latter whether locally (the effect of short columns, node shear failure) or globally (the occurrence of soft floors). Because of its complexity, the positive and negative effects of the infill are still the controversial topic that requires serious attention. Special problem are infilled frames with openings. The dissertation further investigates the influence of openings in the masonry infill using experiments tests and numerical studies. The experimental part of the dissertation consists of tests performed on models of single-storey single-span frame with masonry infill when subjected to constant vertical and horizontal cyclic loads. The program tests involved ten equal specimens of reinforced concrete frame models, with or without openings in the masonry infill. The frames were designed according to European standards, without taking into account the effect of the infill. After their casting they were filled with masonry infill of equal material properties, but with different position, types and opening dimensions, with or without the performance of the vertical confinement around the opening. Performance of specimens, experiments and test results obtained have increased the understanding of the behavior of such structures. Given the necessary limitations of the experimental tests and the necessity of generalization of the conclusions, the experimental results are supplemented with numerical studies, through nonlinear finite element method applied in ATENA ver. 4.3. For the reliability of numerical results, they were first calibrated with the experiments, and then such calibrated models were used to expand the scope of research. Numerical analysis included the openings of other dimensions and position, other dimensions and types of infill material and the amount of transverse reinforcement of the frame. Concluding the results of the overall picture of behavior, influence parameters were obtained for masonry infill with and without openings on the reinforced concrete frame. Understanding of the mechanism and the sequence of the failure will help to take into account the influence of masonry infill with openings in the practice. This eventually enables the use of frame with a masonry infill, with and without opening, as structural element, which will result in greater efficiency and reliability in assessing the its behavior in case of seismic action