I will present the basic approaches currently in use to describe the formation of solid particles ("dust") out of the gas phase in a circumstellar environment. Based on theoretical grounds, different species had been identified as promising candidates to form the primary condensation seeds. In order to quantitatively determine their nucleation rates, thermodynamic and energetic properties, like the Gibbs free energy of formation and the binding energies of the small clusters need to be known. Examples will be given of corresponding cluster structure calculations that allow to derive these data.

Recent observations confirm the presence in the gas phase of some of the required molecules that had not been detected before in the circumstellar environment. On the other hand, high spatial resolution interferometric observations in the mm range seem to basically rule out some of the suspected candidates as being responsible to provide the primary condensate in oxygen-rich circumstellar shells. At the same time, chemical species are favored observationally, whose nucleation process is not yet well understood on the theoretical side.

In the second part of the talk, I will give examples of the kind of nonlinear effects that in general have to be expected in the context of the dust formation process. In the framework of a consistent model for a pulsating, dust forming atmosphere, I will highlight the coupling between the dust component and the physical conditions in the dust forming circumstellar shell. The involved complex regulation circuits make it necessary to describe the dust formation process by a consistent approach that takes these interactions into account.