There exist several vulnerability assessment procedures including code-based detailed analysis methods as well as preliminary assessment techniques which are based on inspection and experience to identify the safety levels of buildings. Various parameters affect the seismic behaviour of buildings, such as dimensions and lay-out of structural members, existence of structural irregularities, presence of soft story or/and weak story, short columns and pounding effects, construction and the workmanship quality, soil conditions, etc. The objective of this study is to examine the effect of four essential structural parameters on the seismic behaviour of existing RC structures by using the most updated analytical tools. The effect of the concrete quality, corrosion effects, short columns and vertical irregularities have been examined. Parametric studies have been carried out on case study real buildings extracted from the Turkish building stock, one of which was totally collapsed in Kocaeli Earthquake of 1999. A control building has been considered for each sample structure with ideal parameters (i.e. without vertical irregularity or good quality of concrete, etc.). Nonlinear static push-over and cyclic analyses have been performed on 2D and 3D models. Base shear versus top displacement curves are obtained for each building in two orthogonal directions. Comparisons have been made in terms of shear strength, energy dissipation capability and ductility. The mean values of the drop in the performance are computed and factors are suggested to be utilized in preliminary assessment techniques, such as the recently proposed P25 method which is shortly summarized in this Chapter.