Inspecting and tuning electric fields on the nanometer scale offers a great potential in overcoming limitations inherent in assembling nanostructures. Both optical and electronic devices may be improved in performance provided that a quantitative knowledge on the strength and orientation of local (stray) fields is gained. Here we present nanoscale investigations of functional surfaces probing the surface potential and electronic properties of ferroelectric and ultra thin organic films. We developed methodologies that are able to non-invasively track the electric field both above and below interfaces, thus providing insight also into the sample. Hence, interface dipole formation and interface charging directly shows up in potential changes revealing the donor/acceptor characteristics of molecules, as well as the surface charge screening in ferroelectrics. Such inspections are possible using conventional scanning force microscopy operated in sophisticated modes measuring the electrostatic force or the inverse piezoelectric effect. Finally, electric fields are also probed in the optical regime using near-field optical methods. Examples are shown where the strength and frequency of surface plasmon resonances become tunable due to simple nanostructuring of metallic thin films.