The doctoral thesis deals with the seismic strengthening of existing structures with a cladding system of cross laminated timber (CLT) plates. An experimental series of monotonic in-plane bending tests of cross laminated timber plates was performed, a quasi-static cyclic testing series of unstrengthened and strengthened unreinforced masonry walls and a series of dynamic tests on a shaking table where a two-story unstrengthened and strengthened reinforced concrete frame with and without masonry infill was analysed. The later was designed according to the demands of the 1981 seismic guidelines and was basically already seismic resistant. The masonry wall tests showed that by using a mechanically connected CLT plate the load bearing capacity can be increased by 34 % and ultimate displacement by 165 %. The RC frame tests with masonry infill showed that the CLT strengthening plates caused the vibration periods of an already damaged structure to return back to their initial state, the building%s torsional flexibility was also decreased. However, the proportion of the strengthening was quite large compared to the basic structure. Numerical models of the tested structures were made, their seismic resistance was calculated. It was found that the proposed CLT cladding strengthening system can reduce the extent of damage on the tested RC frame and frame with masonry infill. A 2D numerical model of a larger unreinforced masonry wall (part of a fictional case study building) that didn%t meet the damage limitation and near collapse limit states, showed that after the instalment of the CLT plates anchored into the floor structure the wall%s seismic resistance was increased. In addition, a parametric study of the influence of boundary conditions on the seismic behaviour of cross laminated timber buildings is performed with different analysis types % a modal analysis with elastic spectrum response, a pushover analysis with the N2 method and a nonlinear dynamic analysis. We have demonstrated how different stiffness, strength and ductility of connections influences on the seismic resistance of CLT structures, the importance of taking the vertical load acting on walls into account as well as the influence of friction. We have also analysed the influence of wall geometry that can be built from either one larger or several smaller pieces. Also, a procedure for the calculation of a wall%s in plane substitute stiffness and strength considering the influence of the vertical load, friction and connections among timber walls and floors is proposed, which is suitable for the engineering practice and allows the definition of a structure%s stiffness for the modal analysis with the response spectra.