Filling a frame (whether reinforced concrete or steel) with an infill wall (whether masonry or concrete) is a common practice in retrofitting existing frames to withstand seismic loads that were not previously anticipated when the frame was designed. This work started with a major experimental program funded by California Transportation Department (CalTrans) and was finalized by the numerical model designed by the author and presented in this book. This numerical model was the benchmark used to perform a parametric analysis studying several factors affecting the properties of the reinforced concrete infill wall. The work presented in this book is the fruit of a research program lead by Professor Medhat Haroun (may his soul rest in peace) through his research group in the University of California Irvine and in the American University in Cairo. The author is indebted to Professor Haroun who was his professor and mentor and prays for God to accept his good deeds and waive his sins.