Recent decades have witnessed considerable research effort in the development and application of the discrete element method (DEM) for modeling granular materials. One key aspect that appears to have not been addressed adequately is the choice or determination of the DEM model parameters to produce satisfactory quantitative predictions. In many DEM studies, the input parameters were often not measured and the values were sometimes assumed without any proper justification. This article describes a careful validation study in which DEM was used to model confined compression of spherical (glass beads) and nonspherical (corn grains) particles. The key properties of the particles were independently measured. The experimental data on the bulk compressibility and the corresponding load transfer to the cylindrical wall and the bottom platen provide quantitative information for direct comparison with the DEM results. Initial particle packing structure is expected to influence the bulk behavior of the assembly, so several methods of particle generation were used to investigate the sensitivity of particle generation in DEM. An investigation of the influence of interparticle friction and particle stiffness in DEM simulations provided an insight into the bulk response of the granular assemblies under confined compression. Comparison between the simulations and the experiments revealed several useful observations, including the roles of these model parameters in the predicted bulk responses.