  Exoplanets with substantial Hydrogen/Helium atmospheres have been discovered
in abundance, many residing extremely close to their parent stars. The extreme
irradiation levels these atmospheres experience causes them to undergo
hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed
to be occurring in a few nearby exoplanet systems through transit spectroscopy
both for hot Jupiters and lower-mass super-Earths/mini-Neptunes. Detailed
hydrodynamic calculations that incorporate radiative transfer and ionization
chemistry are now common in one-dimensional models, and multi-dimensional
calculations that incorporate magnetic-fields and interactions with the
interstellar environment are cutting edge. However, there remains very limited
comparison between simulations and observations. While hot Jupiters experience
atmospheric escape, the mass-loss rates are not high enough to affect their
evolution. However, for lower mass planets atmospheric escape drives and
controls their evolution, sculpting the exoplanet population we observe today.
