A review. Optical spectroscopies provide unique exptl. access to material parameters. In many cases, addnl. information is obtained by measuring sep. tensor elements of the (nonlinear) optical susceptibility tensor. Sepn. of different elements that may represent completely different phys. properties is not always trivial. In this paper a no. of 'sepn. problems' in optical spectroscopy are introduced and analyzed. As exemplary cases we address problems assocd. with interface vs. bulk contributions (in optical second harmonic generation as well as in linear approaches, and both for nonmagnetic and magnetic systems), and the extn. of genuine magnetic information from magneto-optical spectroscopies (both in the static and time-resolved cases). Generic mechanisms and exptl. approaches are identified.