Frequently, the atmospheres of stars of spectral types B to F, on which the attention is particularly focussed in this Joint Discussion, are characterized by non-standard physical conditions: non-solar abundance patterns, surface in homogeneities, strong magnetic fields. The most extreme anomalies are found in the chemically peculiar (CP) stars but more moderate departures from standard atmospheres are observed in many stars in the considered temperature range. This can affect the derivation of fundamental stellar parameters in a number of ways, some of which will be presented in this contribution. The emphasis is set on those physical processes which are specific to CP stars. Due to lack of space, this review is necessarily incomplete: a selection of recent results of interest will be pointed out and some directions of investigation will be suggested.

CP stars have an anomalous continuous energy distribution. Fitting this distribution by standard model atmospheres can only be achieved using models corresponding to different effective temperatures to represent different spectral ranges (e.g., Leone & Catalano 1991). This is obviously not physically meaningful: the effective temperature characterizes the total flux of a star, thus is unique. “Realistic” models must incorporate the anomalous elemental abundances. Such models are presently being developed by Kurucz (ATLAS12) and by the Vienna group (Weiss 1994, private communication). The latter is adapted from Muthsam’s (1979) code. One important result obtained by Muthsam is that the relation T(τ) appears steeper in magnetic Ap stars than in normal stars. This view recently received support from observations of rapid oscillations (Matthews et al. 1990). But even such realistic models may not represent satisfactorily CP star atmospheres, as some potentially important opacity sources may still be unknown. For instance, the broad continuum depressions observed in many magnetic CP stars have not been fully explained yet.