Earthquakes are among the most hazardous actions facing reinforced concrete structures. They can cause cracking in structural members, damage the connections between columns and beams, and, in severe cases, lead to partial or complete collapse. With growing attention to building safety and sustainability, seismic performance assessment has become a central topic in modern structural engineering, particularly for existing buildings that were designed according to requirements that may not meet current seismic codes. What Is Nonlinear Analysis? Nonlinear analysis is an advanced structural analysis approach that accounts for the inelastic behavior of materials and structural members when subjected to loads, such as concrete cracking, yielding of steel reinforcement, and degradation of member stiffness and strength as deformations increase. Unlike linear analysis, which assumes that the structure remains within the elastic range, this approach shows how a building progressively moves from elastic response to various stages of damage under seismic loading. It is therefore closer to the actual behavior of the structure during an earthquake. Main Methods of Nonlinear Analysis Pushover Analysis. In this method, lateral loads are applied incrementally to the structure until a specified level of deformation or collapse is reached, and a capacity curve relating base shear to displacement is derived. This makes it possible to evaluate the building’s strength and deformation capacity, trace the sequence in which plastic hinges form, and identify potential weak zones. The method is relatively easy to apply, and is therefore widely used for the preliminary assessment of existing buildings. Nonlinear Time-History Analysis. This method relies on recorded seismic ground motions to study the building’s response over time, providing information on displacements and inter-story drifts, internal forces in structural members, and the progression of damage during the earthquake. It is regarded as one of the most accurate assessment methods because it represents the dynamic nature of seismic loads. However, it requires greater computational effort and more expertise in model preparation and ground motion selection. The Importance of Nonlinear Analysis Nonlinear analysis gives engineers a greater ability to identify the members most vulnerable to damage, evaluate the structural performance levels of the building, and assess the need for strengthening existing buildings and choose the most suitable retrofit approach. It also helps support design decisions that balance safety requirements and economic cost. Factors Affecting the Accuracy of Results Despite the capabilities this analysis offers, the accuracy of its results depends on several factors, chiefly the quality of the mathematical model and how well it represents the structure’s geometry and reinforcement details; the material properties adopted, including concrete strength and the post-yield behavior of steel reinforcement; the selection of ground motion records in line with the site conditions and its seismic hazard; and the assessment criteria adopted for defining performance levels and acceptance limits. Accordingly, this type of analysis should be carried out by qualified engineers, with careful verification of the model and review of the assumptions. Conclusion Seismic performance assessment using nonlinear analysis is an important scientific tool for understanding the behavior of reinforced concrete buildings under earthquake effects. It supports design and retrofit decisions and enhances the safety and sustainability of structures. Expanding its use in engineering practice and academic research is therefore an essential step toward a built environment that is safer and more resilient to seismic risks. Keywords: Seismic performance, nonlinear analysis, pushover analysis, nonlinear time-history analysis, reinforced concrete structures, building retrofit.