  This paper addresses the issue of how best to correct astronomical data for
the wavelength-dependent effects of Galactic interstellar extinction. The main
general features of extinction from the IR through the UV are reviewed, along
with the nature of observed spatial variations. The enormous range of
extinction properties found in the Galaxy, particularly in the UV spectral
region, is illustrated. Fortunately, there are some tight constraints on the
wavelength dependence of extinction and some general correlations between
extinction curve shape and interstellar environment. These relationships
provide some guidance for correcting data for the effects of extinction.
Several strategies for dereddening are discussed along with estimates of the
uncertainties inherent in each method. In the Appendix, a new derivation of the
wavelength dependence of an average Galactic extinction curve from the IR
through the UV is presented, along with a new estimate of how this extinction
law varies with the parameter R = A(V)/E(B-V). These curves represent the true
monochromatic wavelength dependence of extinction and, as such, are suitable
for dereddening IR--UV spectrophotometric data of any resolution, and can be
used to derive extinction relations for any photometry system.
