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The evolution over time of the magnetic activity and the resulting X-ray andUV coronal and chromospheric emissions of main-sequence dG, dK, and dM starswith widely different ages are discussed. Young cool stars spin rapidly andhave correspondingly very robust magnetic dynamos and strong coronal andchromospheric X-ray - UV (XUV) emissions. However, these stars spin-down withtime as they lose angular momentum via magnetized winds and their magneticgenerated activity and emissions significantly decrease. Studies of dK-dM starsover a wide range of ages and rotations show similar (but not identical)behavior. Particular emphasis is given to discussing the effects that XUVemissions have on the atmospheres and evolution of solar system planets as wellas the increasing number of extrasolar planets found hosted by dG-dM stars. Theresults from modeling the early atmospheres of Venus, Earth and Mars usingrecently determined XUV irradiances and inferred winds of the young Sun arealso briefly discussed. For example, the loss of water from juvenile Venus andMars can be explained by action of the strong XUV emissions and robust winds ofthe young Sun. We also examine the effects of strong X-ray and UV coronal andchromospheric emissions (and frequent flares) that dM stars have on possibleplanets orbiting within their shrunken habitable zones (HZs) - located close tothe low luminosity host stars (HZ < ~0.4 AU). Dwarf M stars make interestingtargets for further study because of their deep outer convective zones (CZs),efficient dynamos, frequent flares and strong XUV emissions. Furthermore, alarge fraction of dM stars are very old (>5 Gyr), which present intriguingpossibilities for the development of highly advanced modes of intelligent lifeon planets that may orbit them.

Evolution over Time of Magentic Dynamo Driven UV & X-ray Emissions of dG-M Stars and Effects on Hosted Planets