In the ’90s DEM simulation research was successful in exploring its potential in the simulation of fluidization phenomena and in its application to the design of fluidized-bed processes. Nevertheless, not much progress has been made regarding the realistic treatment of collision processes that are critical in determining macroscopic mode of fluidization. All through the second half of the ’90s, the author investigated/demonstrated the issue by introducing different surface interactions caused by formation of liquid and/or solid bridges, van der Waals force and the existence of surface roughness. In the first part of the presentation these are to be summarized and the tasks remaining are discussed. In the second part, are presented the results from a newly developed force-deformation meter to demonstrate the significance of the surface roughness and its elasto-plastic characteristics. In the ’90s DEM simulation research was successful in exploring its potential in the simulation of fluidization phenomena and in its application to the design of fluidized-bed processes. Nevertheless, not much progress has been made regarding the realistic treatment of collision processes that are critical in through the second half of the ’90s, the author investigated / demonstrated the issue by introducing different surface interactions caused by formation of liquid and / or solid bridges, van der Waals force and the existence of surface roughness In the first part of the presentation these are to be summarized and the tasks remaining are discussed. In the second part, are presented the results from a newly developed force-deformation meter to demonstrate the significance of the surface roughness and its elasto-plastic characteristics.