Host stars are the major, if not the main, energy input for their surrounding planetary bodies. The star's magnetic fields can vary dramatically, resulting in significant changes to the form and amount of energy inserted into planetary atmospheres and magnetospheres. Continuous observations of the Sun with SDO, and other complementary observatories, provide detailed information regarding the only star that can be studied up close. Data from planetary missions, such as MAVEN, can be used to study the planetary response. CoRoT, Kepler, and -- soon -- TESS, provide data about cool stars of various ages, masses, rotation rates, and activity characteristics that place the Sun in context. Topics include, but are not limited to: magnetic activity on cool stars (including the sun), insights provided by stellar flares, atmospheric escape driven by solar/stellar activity, impacts of solar eruptive events on planetary bodies, and solar wind and cosmic ray modulation.