  Clouds and hazes are commonplace in the atmospheres of solar system planets
and are likely ubiquitous in the atmospheres of extrasolar planets as well.
Clouds affect every aspect of a planetary atmosphere, from the transport of
radiation, to atmospheric chemistry, to dynamics and they influence - if not
control - aspects such as surface temperature and habitability. In this review
we aim to provide an introduction to the role and properties of clouds in
exoplanetary atmospheres. We consider the role clouds play in influencing the
spectra of planets as well as their habitability and detectability. We briefly
summarize how clouds are treated in terrestrial climate models and consider the
far simpler approaches that have been taken so far to model exoplanet clouds,
the evidence for which we also review. Since clouds play a major role in the
atmospheres of certain classes of brown dwarfs we briefly discuss brown dwarf
cloud modeling as well. We also review how the scattering and extinction
efficiencies of cloud particles may be approximated in certain limiting cases
of small and large particles in order to facilitate physical understanding.
Since clouds play such important roles in planetary atmospheres, cloud modeling
may well prove to be the limiting factor in our ability to interpret future
observations of extrasolar planets.
