  The architecture of many exoplanetary systems is different from the solar
system, with exoplanets being in close orbits around their host stars and
having orbital periods of only a few days. We can expect interactions between
the star and the exoplanet for such systems that are similar to the tidal
interactions observed in close stellar binary systems. For the exoplanet, tidal
interaction can lead to circularization of its orbit and the synchronization of
its rotational and orbital period. For the host star, it has long been
speculated if significant angular momentum transfer can take place between the
planetary orbit and the stellar rotation. In the case of the Earth-Moon system,
such tidal interaction has led to an increasing distance between Earth and
Moon. For stars with Hot Jupiters, where the orbital period of the exoplanet is
typically shorter than the stellar rotation period, one expects a decreasing
semimajor axis for the planet and enhanced stellar rotation, leading to
increased stellar activity. Also excess turbulence in the stellar convective
zone due to rising and subsiding tidal bulges may change the magnetic activity
we observe for the host star. Here I review recent observational results on
stellar activity and tidal interaction in the presence of close-in exoplanets,
and discuss the effects of enhanced stellar activity on the exoplanets in such
systems.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics