  Stellar activity due to different processes (magnetic activity, photospheric
flows) affects the measurement of radial velocities (RV). Radial velocities
have been widely used to detect exoplanets, although the stellar signal
significantly impacts the detection and characterisation performance,
especially for low mass planets. On the other hand, RV time series are also
very rich in information on stellar processes. In this lecture, I review the
context of RV observations, describe how radial velocities are measured, and
the properties of typical observations. I present the challenges represented by
stellar activity for exoplanet studies, and describe the processes at play.
Finally, I review the approaches which have been developed, including
observations and simulations, as well as solar and stellar comparisons.
