Abstract: Our Sun is known to harbour magnetic fields which cause myriad of phenomena, e.g., starspots, flares, and coronal mass ejections. The existence of starspots in other stars than the Sun has been known for more than five decades. The development of observing and analysis techniques that has occurred during the past two decades has enabled us to study the detailed starspot and magnetic field configurations on stars of different types and ages. Starspots also change the shape of the observed spectral line profiles in a way that can mimic radial velocity variations from exoplanet, especially hampering the detection of small exoplanets. Additionally, coronal mass ejections, explosive events that occur basically daily on the Sun, are one of the main ingredients in the changing space environment of the Earth. The lack of knowledge on the properties of stellar coronal mass ejections hinders our understanding of the space environments of planets orbiting other stars. In this talk I will discuss stellar activity, especially starspots and coronal mass ejections, and how it affects the orbiting planets and their detection.