  We study the impact of the recently introduced underspread/overspread
classificationon the spectra of processes with square-integrable covariance
functions. We briefly review the most prominent definitions of a time-varying
power spectrum and point out their limited applicability for {\em general}
nonstationary processes. The time-frequency-parametrized approximation of the
nonstationary Wiener filter provides an excellent example for the main
conclusion: It is the class of underspread processeswhere a time--varying power
spectrum can be used in the same manner as the time--invariant power spectrum
of stationary processes.
