Some important aspects of the subject of the repair to and the strengthening of buildings, before and after an earthquake, are investigated in this paper. It has to be noted that, far more than that of recent constructions, many older buildings are susceptible to earthquake damage. It is certain that, from the techno-scientific side of problem alone, the subject is incomparably more difficult and complex than that of designing new structures. The structure of the building has to be dealt with as a whole and the process of redesign includes the following three stages: The assessment of the capacity of the existing structure during seismic activity, the process of decision making and the design of a solution for a pre-selected performance level. The method that will be selected for the intervention will have to be included in the plan of the strategy. This will depend on the required levels of strength, stiffness and deformation of the structure. Practical aspects for six main categories of methods of intervention are discussed in this paper. The six main categories are listed as follows: The addition of infilled walls, the addition of new external walls, the addition of bracing systems, the construction of wing walls, the strengthening of weak elements and the incorporation of energy absorbing systems. In particular, recent results from ongoing experimental research programs concerning the retrofitting of existing structures are presented. Finally, certain important aspects of practical application are pointed out that may often puzzle the engineer or may be solved in an erroneous way. These concern either the choice of the most suitable solution for the intervention or the application of technological methods of intervention.