The presence of dust in galaxies removes one-half or more of the stellar en
ergy from the UV-optical budget of the universe and has a profound impact o
n our understanding of how galaxies evolve. Measures of opacity in local ga
laxies are reviewed together with widely used theoretical and empirical met
hods for quantifying its effects. Existing evidence shows that the dust con
tent of nearby galaxies depends not only on their morphology but also on th
eir luminosity and activity level. A digression is devoted to starbursts in
view of their potential relevance for measures of opacity in distant galax
ies. Scarcity of coherent multiwavelength data sets hampers our ability to
derive reliable obscuration estimates in intermediate- and high-redshift ga
laxies. This, in turn, limits the reliability of inferred physical quantiti
es, such as star formation rates, stellar population ages, galaxy luminosit
y functions, and others.