  The key to understanding an extrasolar giant planet's spectrum--and hence its
detectability and evolution--lies with its atmosphere. Now that direct
observations of thermal emission from extrasolar giant planets are in hand,
atmosphere models can be used to constrain atmospheric composition, thermal
structure, and ultimately the formation and evolution of detected planets. We
review the important physical processes that influence the atmospheric
structure and evolution of extrasolar giant planets and consider what has
already been learned from the first generation of observations and modeling. We
pay particular attention to the roles of cloud structure, metallicity, and
atmospheric chemistry in affecting detectable properties through Spitzer Space
Telescope observations of the transiting giant planets. Our review stresses the
uncertainties that ultimately limit our ability to interpret EGP observations.
Finally we will conclude with a look to the future as characterization of
multiple individual planets in a single stellar system leads to the study of
comparative planetary architectures.
