Exoplanets have intimate relationships with their host stars as the Earth does with its host star. While the Earth’s environment produced by the Sun is usually benign, exoplanets located close to their host stars, especially active M dwarfs, must suffer through powerful flares, stellar winds, CMEs, and very high energy radiation. The environment in which exoplanets must live is now called “stellar space weather” in analogy with the extensively monitored “space weather” that is the environment of the Earth. In this chapter I describe flares and superflares on the Sun and stars and how repeated flares destroy O \({ }_3\) in the atmospheres of exoplanets possibly leading to the sterilization of their surfaces and loss of habitability.