Reinforced concrete (RC) frame structures are dominantly used for the construction of low and medium{high structures on the territory of Croatia, and sometimes such structures in seismically active zones need to be strengthened to increase the lateral bearing capacity, stiness and/or ductility of the primary bearing structure. In this dissertation, the use of masonry inlls, with dierent types of connection between the inll and the RC frame, was elaborated as an element for strengthening the basic RC structure for the earthquake action. It is known that the presence of inlls, and without any additional connection with the frame, signicantly changes the dynamic response of the structure, whereby the inll elements can not be neglected in the dynamic or static analysis of the structure. Existing valid regulations do not contain any clarication of the impact of such reinforced structures, which makes this area even more interesting. The present ndings have shown that the uniform presence of masonry inlls contributes to greater stiness and lateral (in-plane) bearing capacity, with the structure experiencing less global displacements, and thus less damage, which is often considered to be positive eects of masonry inlls. On the other hand, the presence of inlls can lead to local or global plastic deformations of the primary frame structure, forming short columns, shear failures of the beam-column joints or forming the so-called soft storey mechanisms, and reduction of the ductility of individual elements and/or structure as a whole, which are certainly one of the negative eects of inlls. By strengthening the frame by adding inlls in combination with mutual interconnection, it is primarily intended to improve the interaction between the inll and the frame, which can be observed also through the contribution of the ductility of the system, and by reducing the possible negative eects of the inll mentioned earlier, on the one-bay, one-storey specimens. The doctoral dissertation deals with the behaviour of bare planar RC frames, as well as the planar RC frames strengthened by the addition of masonry inlls in combination with the mutual interconnection of the inll and the frame elements for the earthquake action. Using the present knowledge of the composite inlled{frame system and their interaction, the primary aim of this dissertation was to explore the possibilities of using the inll walls in RC frames as a strengthening method, observing the in-plane cyclic responses obtained by experimental testing of particular specimens. In order to contribute to understanding the strengthening of the frame by masonry inlls, experimental and numerical researches have been carried out. The experimental part of the study was carried out on 10 one{bay, one-storey specimens of ductile and non-ductile RC frames strengthened with masonry inll (hollow clay block and solid clay bricks units) and 7 related reference specimens used for evaluation of inll strengthening and the quality of the inll and frame mutual interconnection. In addition to the static cyclic test of specimens itself, the mechanical characteristics of the embedded materials were also tested (concrete, reinforcement, wall elements, masonry, mortar and composite materials). Frame specimens are built in scale 1:2.5 for ductile and 1:2 for non-ductile frames, tested at constant vertical and incrementally increased horizontal cyclic load (static reversed-cyclic test). The test specimens can be grouped according to the type of inll element (hollow clay block and solid clay brick units), type of RC frame (ductile and non-ductile), and type of connection between the inll and the frame. This provides a total of 6 diferent strengthening congurations, of which some are applied to two types of inll. By analysing the results of these experimental studies, the characteristics of the behaviour of each specimen were presented, and the most suitable type of strengthening for the ductile and non-ductile frames was determined. For all experimental specimens, of which 10 original and 7 used as references, the same calibration was carried out using nonlinear macro-models in OpenSees (PEER, Berkeley, opensees.berkeley.edu). Numerical models are based on so- called beam-column elements for columns and beams, and specic type of compressive/tensile elements for inll walls, for which satisfactory responses to cyclic responses have been achieved with respect to experiments. The same models were used in the numerical case study for predicting the dynamic response to the application on a real building (Tsukuba, Japan) by Incremental Dynamic Analysis (IDA), where IDA curves, fragility curves, vulnerability curves and loss curves were designed for each type of strengthening separately to evaluate the eciency of a particular strengthening in relation to reference models. Finally, an experimental case study of the spatial FRAMA building, Model #1 and Model #2 (FRAmed-MAsonry Composites for Modelling and Standardization, framed-masonry.com), which was built in scale of 1:2.5 and dynamically tested shaking table at the IZIIS (Institute of Earthquake Engineering and Engineering Seismology, iziis.edu.mk), Skopje, Macedonia. The FRAMA building, Model #1 and #2, have very similar characteristics in geometry, type of inll and strengthening type, as is the case with some original planar experimental specimens from the first part of the research. FRAMA Model #1 represents the reference RC frame with masonry inlls of a hollow clay block, without additional mutual interconnections between the inlls and the frame, while Model #2 represents a strengthened model derived from a previously tested RC frame (Model #1), using a solid clay brick in combination with vertical tie-columns. Global dynamic responses, maximum intermediate drifts, and comparison of fundamental frequency of Model #1 and #2 are shown briefly, as an strengthening evaluation of Model #2 with respect to Model #1.