We have modeled the layer-by-layer assembly process of flexible polyelectrolytes on flat surfaces. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving towards a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. The effects of surface charge density, bulk salt concentration, and solvent quality on the thickness and internal structure of the multilayer are also studied. Our results agree with experimental findings.