Earthquakes are one of the biggest problems in civil engineering all over the world. Due to earthquakes, great disasters in cities with collapsed structures and human losses have been caused. More specific, old buildings that have been built based on old regulations and design building codes do not fulfil anymore the new criteria of seismic designing.In this study, an old building has been evaluated for the seismic load in order to decide if there is a need for strengthening it using retrofitting methods. The seismic evaluation is based on Eurocode 8 and after the application of retrofitting techniques the building fulfilled its seismic design criteria. The existing building is a four-storey, concrete structure that has been built in 1970 and is located in Athens (the capital city of Greece). The seismic evaluation is conducted by using the software Seismostruct.Two analyses are performed in order to evaluate the seismic behavior of the building. First, an eigenvalue analysis is conducted before and after retrofitting. By using this analysis the torsional sensitivity of the building has been checked. Then, using pushover analysis, the comparison of the target displacement (expected displacement of the building for the design seismic action) for each limit state and the displacement of the building when the first member of the building reached the corresponding limit state, is presented. Target displacement must not be greater than this displacement in order to ensure the safety of the building. If the comparison shows that target displacement is greater, the weak links in the facility should be identified and the proper retrofitting method should be applied for the improvement of the seismic behavior of the building. Pushover analysis is conducted before and after the application of retrofitting methods.After performing the eigenvalue and pushover analysis of the existing building it was found that the building was torsional sensitive and shear failures occurred in many beams of the structure. Regarding the bending failures, the target displacement was not greater than the displacement of the building when the first member of the building reached any of the corresponding limit states. Therefore the building was safe against bending failures. With the application of X-shaped steel braces in selected frames, the building had higher stiffness and it was not torsional sensitive but shear failures occurred again in many beams. Furthermore, compressive failures occurred in columns that were connected with the steel braces. Finally, with the application of fibre reinforced plastic jacketing in the members that failed in the previous pushover analysis there were no shear or compressive failures. Finally the structure was safe against seismic actions.The application of retrofitting methods improved the seismic behavior of the building and the structure fulfilled the updated regulations of Eurocode 8 regarding seismic design. This project thesis may give rise to further studies and researches concerning seismic retrofitting and seismic damage prevention.