The present chapter 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. Two examples of photoreactive polymers together with the underlying photochemistry are presented. Polymers bearing aryl ester groups or N-arylamide units in their side-chain undergo the photo-Fries reaction under UV-light which yields hydroxyketone and aminoketone units as photoproducts, respectively. The reaction results in a change in surface polarity, which is accompanied by a significant increase in refractive index (Δn up to +0.10). Another example is given with polymers bearing ortho-nitrobenzyl ester units in their side chain. UV-irradiation causes the scission of the ester unit and the formation of polar carboxylic acids. Employing these photosensitive polymers as interfacial layers between the organic semiconductor and the gate dielectric, the characteristics of OTFTs such as carrier mobility and threshold voltage can be varied over a wide range. Moreover, the epitaxial growth of organic semiconductors (para-sexiphenyl and pentacene) on these surfaces can be influenced by photochemical adjustment of surface polarity. Proceeding from thin polymer layers to molecular layers, several examples for photoreactive mono- and oligolayers on metals and oxidic surfaces are presented. These layers, containing aryl ester units for example, were modified by UV illumination and post-exposure derivatization. Lithographic patterns in molecular layers were characterized with friction force microscopy.