The Sun's magnetic activity has changed considerably during its evolution from a protostar to a low-activity main-sequence star. The young Sun produced orders of magnitude more hot plasma, X-ray emission, and high-energy particles. The present paper summarizes a few recent investigations on the nature of the magnetic activity and associated high-energy processes in the Sun at different stages of its evolution, with emphasis on the physical mechanisms that may be responsible for coronal heating, the large-scale structure of the magnetic fields, and the possible mass-loss history of the Sun.