The modification of materials by monolayers of polymers, which are covalently attached to the surface of the substrate, is a very attractive way to improve the properties of solids in bio-oriented applications. We describe several new pathways for the synthesis of surface-attached ultrathin polymer films, which carry functional groups relevant for biological or biomedical applications. The polymer molecules are either grown at the surface of the substrate in situ by using self-assembled monolayers of initiators or preformed polymers are (photo-)chemically attached to the material, which is to be modified. Additionally, the formation of ultrathin, patterned networks of functional polymers will be described. Examples for groups contained in the monolayers are peptide moieties, which could act as cell recognition sites and DNA fragments for biochip applications. The characterization of the monolayers, especially the swelling of the layers in an aqueous environment, will be described.