  Understanding the properties and physical mechanisms that shape dust
attenuation curves in galaxies is one of the fundamental questions of
extragalactic astrophysics, with a great practical significance for deriving
the physical properties of galaxies, such as the star formation rate and
stellar mass. Attenuation curves result from a combination of dust grain
properties, dust content, and the spatial arrangement of dust and different
populations of stars. In this review we assess the current state of the field,
paying particular attention to the importance of extinction curves as the
building blocks of attenuation laws. We introduce a quantitative framework to
characterize and compare extinction and attenuation curves, present a
theoretical foundation for interpreting empirical results, overview an array of
observational methods, and review the observational state of the field at both
low and high redshift. Our main conclusions are: Attenuation curves exhibit a
large range of slopes, from curves with shallow (Milky Way-like) slopes to
those exceeding the slope of the SMC extinction curve. The slopes of the curves
correlate strongly with the effective optical opacities, in the sense that
galaxies with low dust column density (lower visual attenuation) tend to have
steeper slopes, whereas the galaxies with high dust column density have
shallower (grey) slopes. Galaxies appear to exhibit a diverse range of 2175A UV
bump strengths, but on average have suppressed bumps compared to the average
Milky Way extinction curve. Theoretical studies indicate that variations in
bump strength may result from similar geometric and radiative transfer effects
that drive the correlation between the slope and the dust column.

Astrophysics of Galaxies