  We are using optical/IR surface brightness fluctuations (SBFs) to validate
the latest stellar population synthesis models and to understand the stellar
populations of ellipticals. Integrated light and spectra measure only the first
moment of the stellar luminosity function (\Sigma n_i * L_i). Since SBFs also
depend on the second moment (\Sigma n_i * L_i^2), they provide novel
information, in particular about the reddest, most luminous RGB and AGB stars,
which are the most difficult stars to model. SBFs can also provide useful new
constraints on the age/metallicity of unresolved stellar populations in
ellipticals. Finally, developing accurate stellar population models benefits
several aspects of SBF distance measurements to galaxies.
