AbstractThe changing environment of telecommunications service operators leads to an increasing customer orientation and an ever increasing complexity. The technical consequence of the increased customer orientation is visible in the higher interest in flexibility engineering. The increasing complexity is becoming the major problem in telecommunication systems of this decade. To deal with this complexity two fundamental capabilities are used: abstraction and structuring. In general, models can be identified on any level of abstraction, e.g. high‐level system behaviour models, high‐level system architectural models and low‐level architectural models. When system complexity increases, more levels of abstraction will be necessary to deal with this complexity. Various methods exist to define models at different levels of abstraction. In this paper three modelling methods are discussed. It is demonstrated that abstraction is used in a similar way in these modelling methods. One particular method for abstraction is the use of the object‐oriented paradigm, a powerful approach for modelling as well as for implementation. Additional structuring methods based on considerations with respect to the functionality, however, remain indispensable in order to keep complexity under control. Those methods, stratification and segregation, are specifically relevant in high level architectural