Whether an exoplanet retains its atmosphere and surface water depends on many factors. I will first describe the two main mass-loss mechanisms for exoplanet atmospheres. The main thermal process is hydrodynamical outflow driven by extreme ultraviolet and X-radiation from the host star that heat and inflate an exoplanet’s exosphere leading to mass loss. Non-thermal mass loss occurs when ions and electrons in the host star’s wind remove neutrals and ions in the exoplanet’s upper atmosphere through charge exchange, ion pick-up, and other processes. Although there are simulations of thermal and non-thermal processes occurring in isolation, these processes can work together to enhance the mass-loss rate.