Recent major earthquakes around the world have evidenced that research in earthquake engineering must be directed to the assessment of vulnerability of existing constructions lacking appropriate seismic resisting characteristics. Their retrofit or replacement should be made in order to reduce vulnerability, and consequent risk, to currently accepted levels. The development of retrofitting techniques represents a key issue in order to avoid both human casualties and economic losses. The aims of this thesis are to experimentally study the behaviour of existing reinforced concrete buildings, to calibrate a refined numerical model in order to reproduce rigorously their structural behaviour, to develop a simplified methodology for non-linear dynamic analysis of irregular buildings, and to propose a methodology for optimum strengthening. As a background, a theoretical summary on the most common causes of damage and failure and on repair and strengthening techniques for existing reinforced concrete buildings is presented. Older buildings, designed and constructed until the late 1970's, without considering earthquake provisions, constitute a significant hazard in many cities of southern Europe. Two full-scale four-storey frame models, representative of the common practice of construction until the late 1970's in most southern European countries, were designed, constructed and tested pseudo-dynamically. This experimental study aimed at assessing the original capacity of existing structures, with and without infill masonry, and to compare performances of different retrofitting solutions. The tests have shown that the vulnerability of existing reinforced concrete frames designed without specific seismic resisting characteristics, which are an important part of the existing buildings in Europe, constitute a source of high risk for human life. Furthermore, it was demonstrated that advanced retrofitting methods, solutions and techniques substantially reduce that risk to levels currently considered in modern design. Refined finite element models for the frame and infill masonry were calibrated with the results of the full-scale tests. Special attention was devoted to bond-slip phenomenon, which is likely to influence the behaviour of existing reinforced concrete structures with round smooth reinforcing. The improved models were found capable to analyse existing reinforced concrete structures, reproducing accurately their non-linear response. Additionally, it is proposed a simplified methodology for non-linear dynamic analysis of buildings based on the multi-modal spectral seismic response. This methodology is a valuable tool to analyse irregular structures and constitutes an important tool for the optimum strengthening design of existing buildings, which was also developed in this thesis. ; JRC.G.5-European laboratory for structural assessment