  Model-predicted and observed properties of the brightness distribution on
M-type Mira disks are discussed. Fundamental issues of limb-darkening and
diameter definition, of assigning observational data to diameter-type
quantities and of interpreting such quantities in terms of model diameters are
outlined. The influence of model properties upon interpretation of measured
data is clarified. The dependence of the centre-to-limb variation (CLV) of
intensity on wavelength, on stellar parameters and on variablity phase and
cycle may be used for analyzing the geometrical and physical structure of the
Mira atmosphere, for determining fundamental stellar parameters, and for
investigating the quality of models. Desirable future observations include
simultaneous observations in different spectral features at different phases
and cycles, observation of the position of the shock front and observation of
the time- and wavelength-dependence of deviations from spherical symmetry.
