In addition to photons at all wavelengths, stars eject ionized and neutral particles to space. These typically magnetized winds interact with the upper layers of exoplanet atmospheres producing charge-exchange and other reactions that facilitate mass loss. In this chapter, I describe the different types of winds observed in hot and cool stars. For some star types, it is relatively easy to measure diagnostics of mass loss but in solar type stars it is more difficult. I describe the different techniques for measuring or estimating stellar mass-loss rates and the diverse models developed to explain stellar mass loss.