In this chapter I would like to present some examples for the great success of neutron and x-ray reflectometry in polymer science. These techniques are unique for the determination of interfacial density profiles, even of buried interfaces. The vertical resolution of these techniques is at least comparable with SFM (scanning force microscopy) but at the same time they take averages over large enough areas to give a representative and characteristic information of the system. Other techniques may be more direct (like SFM or NRA (nuclear reaction analysis)) but they have severe disadvantages concerning either vertical resolution or lateral sampling. While SFM may provide detailed information on small sample sizes this may not be representative for the whole sample. NRA and other ion beam techniques are certainly more direct as they work in direct space (and not in Fourier space as neutron and x-ray reflectivity). However, their vertical resolution is in many cases insufficient to detect all important features of polymeric interfaces. Comparing the vertical depth resolution of neutron and x-ray reflectometry (of the order of Angstroms) to the typical size of a polymer (some hundreds of Angstroms) clearly shows the possibility to measure changes at a submolecular level.