The present work deals with photoreactive thin films and describes processes to tune both surface and material properties by means of UV-irradiation. Selected applications of these materials as UV-tunable interfaces in organic electronics are demonstrated. Examples of photoreactive poly(norbornenes) together with the underlying synthesis and photochemistry are presented. Upon exposure to UV-light polymers bearing ortho-nitrobenzyl ester units in their side chains undergo the scission of the ester unit and polar carboxylic acids are generated. Employing these photosensitive polymers as interfacial layers between an organic semiconductor and the gate dielectric, characteristics of organic thin film transistors (OTFTs) such as carrier mobility and threshold voltage could be varied over a wide range. Moreover, the epitaxial growth of organic semiconductors (para-sexiphenyl and pentacene) on these surfaces was influenced by the photochemical adjustment of surface polarity. The photo induced modulation of surface polarity was accompanied by a significant change in the refractive index (n up to 0.047). Copolymers bearing ortho-nitrobenzyl ester moieties and aryl ester units (photo-Fries rearrangement) in their side chains allowed wavelength-selective tuning, patterning and even inverting of the refractive index. Proceeding from thin polymer layers to molecular layers silane based bifunctional molecules forming photoreactive mono- and oligolayers on metals and oxidic surfaces are presented. These layers, containing ortho-nitrobenzyl ester units, were modified by UV-illumination and post-exposure derivatization. Lithographic patterns in molecular layers were characterized with friction force microscopy (FFM). Furthermore, a novel polyaniline derivative bearing photosensitive N-formamide groups is demonstrated. Via UV-illumination a decarbonylation reaction resulting in polyaniline was introduced that was subsequently protonated to yield the conductive emeraldine salt. These photoinduced conductivity changes were corroborated in thin films by conductive AFM (CAFM) measurements. In addition, the application as photopatternable charge injection layer for structured OLEDs is demonstrated.