  The phenomenon of spectral mimicry refers to the fact that hypergiants and
post-AGB supergiants - stars of different masses in fundamentally different
stages of their evolution have similar optical spectra, and also share certain
other characteristics (unstable and extended atmospheres, expanding gas-dust
envelopes, high IR excesses). As a consequence, it is not always possible to
distinguish post-AGB stars from hypergiants based on individual spectral
observations in the optical range. Examples of spectral mimicry are presented
using uniform, high-quality spectral material obtained on the 6-m telescope of
the Special Astrophysical Observatory in the course of long-term monitoring of
high-luminosity stars. It is shown that unambiguously resolving the mimicry
problem for individual stars requires the determination of a whole set of
parameters: luminosity, wind parameters, spectral energy distribution, spectral
features, velocity field in the atmosphere and circumstellar medium, behavior
of the parameters with time, and the abundances of chemical elements in the
atmosphere.
