In the thesis, the behaviour of three-leaf stone masonry walls under compression loading and in-plane shear loading is studied. An extensive experimental campaign was conducted on 18 walls, accompanied by tests on masonry constituents (mortar and stone). The type of the tested masonry is typical for older representative buildings, which often present important cultural heritage assets. Influence of morphology, level of pre-compression and boundary conditions on various characteristics of the walls behaviour was systematically studied. Besides the strength, the greatest emphasis was on the analysis of the displacement capacity and damage of the walls at characteristic stages of their response. Due to various boundary conditions, different failure mechanisms developed; rocking, mixed and diagonal shear. Leaf separation and the out-of-plane mechanism of the wall was not critical as expected. It developed more evidently in the post-peak phase of the tests for specimens with higher pre-compression. The presence of connecting stones had no influence neither on the obtained shear strength nor on the displacement capacity of the walls. The experimental results were compared to results of analytical models for prediction of shear resistance. For the tested type of masonry, shear strength can be adequately estimated with existing models for the failure mechanisms that developed in the tests. Drift capacity of the walls was however significantly higher than drift capacity allowed in the code provisions (EN 1998-3 in FEMA 306). In heritage buildings also various artistic assets in the form of painted walls are often present. Lime plaster was applied to the walls in order to study its performance during cyclic shear loading. Reference drift values for walls at 4 different characteristic plaster damage states were determined. They can be used for performance based seismic assessment of historic buildings. The second part of the thesis deals with strengthening of the damaged three-leaf stone masonry walls. For monumental buildings there are usually strict demands upon the use of materials compatible to existing materials, reversibility, etc., therefore a new strengthening system was developed. Walls were retrofitted along the cracks with lime-cement grout, additionally strengthened with near surface mounted (NSM) glass cords and transversally connected; 10 walls were strengthened with various combinations of measures and re-tested. Grouting successfully retrofitted the walls, while NSM glass cords increased the displacement capacity and, in one case, also shear resistance substantially. Finally, the results of tests of the un-strengthened walls (drifts) were adopted for the numerical analysis of the seismic performance of an actual building (mansion Vipolže). Nonlinear static analyses using equivalent frame model were conducted. The influence of the assumed drift limits on the seismic resistance was analysed. The increase of seismic performance with increasing drift limits of walls is evident and it would be reasonable to further study the prospect of increasing the drift limits in the code provisions for types of masonry, which are more ductile (historical masonry).