  I review the basic properties of interstellar extinction in the Milky Way
galaxy, focusing primarily on the wavelength dependence within the IR through
UV spectral region. My primary goal is to review the evidence supporting the
idea that Galactic extinction curves can be considered a 1-parameter family
characterized by their value of R(V) = A(V)/E(B-V). Based on analysis of new
(i.e., 2MASS) and old (i.e., IUE) data for about 100 sightlines, I show that
the UV, optical, and IR wavelength regimes do display coherent variations, but
with too much intrinsic scatter to be considered truly correlated. A
1-parameter family can be constructed which illustrates these broad trends, but
very few individual sightlines are actually well-reproduced by such a family
and disagreement with the mean trends is not a sufficient condition for
``peculiarity.'' Only a very small number of extinction sightlines stand out as
truly peculiar. It is likely that simple variations in the mean grain size from
sightline to sightline are responsible for much of the coherent variability
seen in Galactic extinction, and might also explain the ``peculiar'' extinction
long-noted in the Magellanic Clouds.
