The adsorption behavior of positively charged poly(propylene imine) dendrimers on glass has been studied by scanning angle reflectometry as a function of generation, pH, and ionic strength. The results indicate that the adsorption is controlled by properties of both the dendrimers (size and charge) and the surface layer (distribution of glass surface sites). At constant pH and ionic strength the collector properties remain the same, and the adsorption scales with dendrimer size. Because of the limited number of glass surface sites there is only 15% surface coverage at maximum. Adsorption increases with decreasing pH and increasing ionic strength, and the effect of pH is much more pronounced for the smaller dendrimers. To explain the adsorption results, not only the size and charge of the dendrimer have to be taken into account, but also the charge density and the roughness of the surface. This implies that the dendrimer size is comparable to the characteristic length scale of the surface roughness.