We have developed CONCEPTOOL, a system which supports the modelling and the analysis of expressive domain knowledge and thus of different kinds of application ontologies. The knowledge model of CONCEPTOOL includes semantic constructors (e.g. own slots, enumerations, whole-part links, synonyms) separately available in most frame-based, conceptual, and lexical models. Domain knowledge analysis in CONCEPTOOL explicitly takes into account the differences between distinct categories of concepts such as classes, associations, and processes. Moreover, our system uses lexical and heuristic inferences in conjunction with logical deductions based on subsumption in order to improve the quantity and the quality of analysis results. CONCEPTOOL can also perform approximate reasoning by ignoring in a controlled way selected parts of the expressive power of the domain knowledge under analysis.