Early attempts to apply FacetModelling made it clear that tightly integrated multi-perspective multi-usage modelling is difficult without an elaborate and solid semantical foundation. I have therefore recently aligned the ontological assumptions behind facet modelling with the more elaborate Bunge-Wand-Weber (BWW) model of information systems. The BWW-model now supports definition of facet languages as follows: (1) Modelling constructs are first interpreted and defined precisely in relation to the BWW-model. (2) By inspecting their BWW-definitions, ontological overlaps between modelling constructs can be identified at a fine level of granularity. (3) The semantical overlaps are finally resolved by Tre-formulatingT the modelling constructs as parts of a facet modelling language. The combined approach has already been used in the analysis and re-formulation of PPP. Steps (1) and (2) above have even been undertaken for OML and FRISCO. An ontological analysis and evaluation of the Unified Modeling Language (UML) is currently in preparation. Another thread of ontological research looks specifically at the whole-part construct as it is used in OO modelling. While being an important issue within OO-modelling semantics, clearly defined whole-part relations are also a prerequisite for proper integration of perspectives in facet models. (Different perspectives are often expressed at different levels of granularity.)