Any treatment claiming a realistic description of a circumstellar dust shell has to take into account at least five physically and technically rather different complexes:

1. • Hydrodynamics

2. • Thermodynamics

3. • Radiative transfer

4. • Chemistry

5. • Dust condensation

By their combined action, these complexes determine the local physical behavior and global spectral appearance of the circumstellar dust shell. These fundamental complexes and their mutual causal interplay are outlined in Figure 3.1. The coupling indicated by arrows is rather tight, making any consistent realistic modeling an extremely nonlinear problem with regard to both a reliable physical and chemical description and the appropriate mathematical and numerical treatment.

Figure 3.1 displays the general situation for a typical circumstellar dust shell and thus provides a principal frame containing the main ingredients of any reliable approach, yet it does not show which level of description in each box and for each coupling of the different boxes has to be adopted for an appropriate and consistent theoretical and structural modeling of a specific situation. For this aim, the main complexes and their essential physical and chemical couplings, highlighted in this figure, will be outlined in detail in the following chapters.