ABSTRACT. 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.
KEY WORDS: ISM: dust, extinction
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