A novel concept for the synthesis of molecularly thin polyelectrolyte layers covalently attached to planar solid surfaces is introduced. The polymer monolayers are generated directly at the surface by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. This "grafting from" approach yields surface-bound polymer molecules with a high molecular weight and with high graft densities of the attached chains ("brush"-like state). As an example for the preparation of a positively charged polyelectrolyte "brush" at first a monolayer of poly(4-vinylpyridine) (PVP) was generated at the surface, which was subsequently quarternized to yield a poly[4-vinyl-N-n-butylpyridinium] bromide (BuPVP) monolayer. The thickness of this resulting cationic monolayer can be controlled in a wide range, starting from 2 nm to more than 1000 nm in the solvent-free state.