This research project was an effort to improve the understanding of the internal stress-strain distribution in geosynthetic reinforced soil (GRS) retaining structures. The numerical modeling technique utilized a commercially available finite difference program, FLAC (Fast Lagrangian Analysis of Continua). The plane strain soil properties, the effect of low confining pressure on the soil dilation angle, and in-soil and low strain rate geosynthetic reinforcement properties were investigated and appropriately considered in this research. Modeling techniques that are able to predict both the internal and external performance of GRS walls simultaneously were developed. Instrumentation measurements such as wall deflection and reinforcement strain distributions of a number of selected case histories were successfully reproduced by the numerical modeling techniques. Moreover, these techniques were verified by successfully performing true "Class A" predictions of three large-scale experimental walls. An extensive parametric study that included more than 250 numerical models was then performed to investigate the influence of design factors such as soil properties, reinforcement stiffness, and reinforcement spacing on GRS wall performance. Moreover, effects of design options such as toe restraint and structural facing system were examined. An alternative method for internal stress-strain analysis based on the stress-strain behavior of GRS as a composite material was also developed. Finally, the modeling results were used to develop a new technique for predicting GRS wall face deformations and to make recommendations for the internal stability design of GRS walls.