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