  Atmospheric mass loss is a fundamental phenomenon shaping the structure and
evolution of planetary atmospheres. It can engage processes ranging from global
interactions with the host star and large-scale hydrodynamic outflows to
essentially microphysical kinetic effects. The relevance of these processes is
expected to change between planets of different properties and at different
stages in planetary and stellar evolution. The early evolution of planetary
atmospheres, as well as atmospheric escape from close-in planets hosting
hydrogen-dominated atmospheres, is thought to be driven by thermal hydrodynamic
escape, while the kinetic non-thermal effects are most relevant for the
long-term evolution of planets with secondary atmospheres, similar to the inner
planets in the Solar System. The relative input of different mechanisms, hence,
the mass loss rate, shows a complicated dependence on planetary parameters and
the parameters of the host star, where the latter evolve strongly with time. It
results in a large variety of possible evolution paths of planetary
atmospheres.
