The calculation of spin susceptibilities in metals, in particular the Ruderman-Kittel-Kasuya-Yosida (RKKY) polarization due to a point coupling, is reviewed. In the one dimensional case, the low coupling limit of the non-linear theory clarifies traditional perturbation approaches. Multilayer structures require theories with bounded metals. An explicit formula is derived for the indirect interaction between two ferromagnetic plates in a half space. A case, where an oscillatory amplitude decays with the first power of the distance to the boundary is discussed.