Most young late-type stars are rapid rotators, because they have the
primordial rotation rates induced by the interstellar molecular cloud from
which they were formed. Also their older counterparts in close binary systems
can show very high rotation rates. Rapid rotation enhances the dynamo
operation, and thus also introduces significantly stronger magnetic activity
than is seen in slower rotators. This activity  manifests itself, for example,
as large starspots, strong flares and possibly numerous coronal mass ejections.
This is the environment in which planets are formed, and the activity could
have a significant impact on the formation, evolution and habitability of the
planetary systems. In addition, stellar activity can have effects on stellar
spectral lines that mimic radial velocity changes from an orbiting planet,
making it at times difficult to distinguish between planets and activity
signatures, and especially hampering the detection of small Earth-sized planets
using radial velocity method. In this talk I will discuss stellar magnetic
activity, how we study it, what we know about it, and how it can affect
exoplanets and their detection.