  Interstellar dust plays a central role in shaping the detailed structure of
the interstellar medium, thus strongly influencing star formation and galaxy
evolution. Dust extinction provides one of the main pillars of our
understanding of interstellar dust while also often being one of the limiting
factors when interpreting observations of distant objects, including resolved
and unresolved galaxies. The ultraviolet (UV) and mid-infrared (MIR) wavelength
regimes exhibit features of the main components of dust, carbonaceous and
silicate materials, and therefore provide the most fruitful avenue for detailed
extinction curve studies. Our current picture of extinction curves is strongly
biased to nearby regions in the Milky Way. The small number of UV extinction
curves measured in the Local Group (mainly Magellanic Clouds) clearly indicates
that the range of dust properties is significantly broader than those inferred
from the UV extinction characteristics of local regions of the Milky Way.
Obtaining statistically significant samples of UV and MIR extinction
measurements for all the dusty Local Group galaxies will provide, for the first
time, a basis for understanding dust grains over a wide range of environments.
Obtaining such observations requires sensitive medium-band UV, blue-optical,
and mid-IR imaging and followup R ~ 1000 spectroscopy of thousands of sources.
Such a census will revolutionize our understanding of the dependence of dust
properties on local environment providing both an empirical description of the
effects of dust on observations as well as strong constraints on dust grain and
evolution models.
