  Surface Brightness Fluctuations (SBFs) are one of the most powerful
techniques to measure the distance and to constrain the unresolved stellar
content of extragalactic systems. For a given bandpass, the absolute SBF
magnitude \bar{M} depends on the properties of the underlying stellar
population. Multi-band SBFs allow scientists to probe different stages of the
stellar evolution: UV and blue wavelength band SBFs are sensitive to the
evolution of stars within the hot Horizontal Branch (HB) and post-Asymptotic
Giant Branch (post-AGB) phase, whereas optical SBF magnitudes explore the stars
within the Red Giant Branch (RGB) and HB regime. Near- and Far-infrared SBF
luminosities probe the important stellar evolution stage within the AGB and
Thermally-Pulsating Asymptotic Giant Branch (TP-AGB) phase. Since the first
successful application by Tonry and Schneider, a multiplicity of works have
used this method to expand the distance scale up to 150 Mpc and beyond. This
article gives a historical background of distance measurements, reviews the
basic concepts of the SBF technique, presents a broad sample of these
investigations and discusses possible selection effects, biases, and
limitations of the method. In particular, exciting new developments and
improvements in the field of stellar population synthesis are discussed that
are essential to understand the physics and properties of the populations in
unresolved stellar systems. Further, promising future directions of the SBF
technique are presented. With new upcoming space-based satellites such as Gaia,
the SBF method will remain as one of the most important tools to derive
distances to galaxies with unprecedented accuracy and to give detailed insights
into the stellar content of globular clusters and galaxies.
