  A planetary atmosphere is the outer gas layer of a planet. Besides its
scientific significance among the first and most accessible planetary layers
observed from space, it is closely connected with planetary formation and
evolution, surface and interior processes, and habitability of planets. Current
theories of the planetary atmosphere were primarily obtained through the
studies of eight large planets, Pluto and three large moons (Io, Titan, and
Triton) in the Solar System. Outside the Solar System, more than four thousand
extra-solar planets (exoplanets) and two thousand brown dwarfs have been
confirmed in our galaxy, and their population is rapidly growing. The rich
information from these exotic bodies offers a database to test, in a
statistical sense, the fundamental theories of planetary climates. Here we
review the current knowledge of atmospheres of exoplanets and brown dwarfs from
recent observations and theories. This review highlights important regimes and
statistical trends in an ensemble of atmospheres as an initial step towards
fully characterizing diverse substellar atmospheres, that illustrates the
underlying principles and critical problems. Insights are obtained through
analysis of the dependence of atmospheric characteristics on basic planetary
parameters. Dominant processes that influence atmospheric stability, energy
transport, temperature, composition, and flow pattern are discussed and
elaborated with simple scaling laws. We dedicate this review to Dr. Adam P.
Showman (1968-2020) in recognition of his fundamental contribution to the
understanding of atmospheric dynamics on giant planets, exoplanets, and brown
dwarfs.
