Résumé
This chapter provides an overview of the magnetic activity of the Sun and stars, discussing its underlying physical origin, manifestations, and fundamental role in exoplanet studies. It begins with a summary of the Sun's magnetic activity from the surface towards the outer atmospheric layers, highlighting features such as sunspots, faculae, chromospheric structures, and their temporal modulation known as the activity cycle. These phenomena are sustained throughout the lifetime of the Sun by the magnetic dynamo, which is driven by differential rotation and convective flows. Furthermore, extending these concepts to other stars, the chapter examines the diagnostics that are typically employed to track and quantify the magnetic activity level of stars, and it reviews spectropolarimetry, an observational technique with which to characterise stellar magnetic fields. We finally outline results from both observations and theoretical modelling of stellar activity across distinct spectral types, and we describe the variety of methods used to search for stellar activity cycles, underscoring the multi-wavelength nature of this field of research.