Magnetic activity is a prime diagnostic for the stellar dynamo and as such an indirect tracer for magnetic fields that are very hard to measure in stars. Studies of the highly energetic and dynamic radiation from the outer atmospheres of late-type stars has received an extra boost during the last few years when its relevance for the evolution of (exo)planets was recognized. In this talk I describe some recent and ongoing multi-wavelength observational projects that address different aspects of stellar activity involving a range of stars from solar-type over the prototypical M dwarf flare stars to the brown dwarf regime. The topics include the relation of activity with stellar rotation and age, the physics of flares, the onset of activity cycles and the effects of stellar activity on potential planets. I also include a brief introduction to the instruments used for these studies, including photometric space missions (Kepler, K2 and TESS) and the latest X-ray survey instrument, eROSITA. Prof. Stelzer will be based at the MPIAstronomie for her visit to Heidelberg and will be available for meetings by arrangement with her host, Wolfgang Brandner (brandner@mpia.de). Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://zoom.us/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09