In Chaps. 10 and 11 , I surveyed the various ways in which a host star’s radiation and wind can erode an exoplanet’s atmosphere, change its chemistry, and thereby determine whether the exoplanet could be habitable. I now turn to the question of whether an exoplanet can change the properties of its host star, in particular, its rotation rate, UV and X-ray radiation, and the properties of its wind. The study of this feedback of an exoplanet on its host star is usually called star-planet interactions (SPI), although a more accurate term would be planet-star interactions.