Most of us are aware that the Sun influences the weather on Earth. The Sun also has a profound impact on our upper atmosphere and the magnetic field that surrounds our planet. Ejections of particles and energy from the Sun sometimes set off electrical disturbances in this magnetic field that affect sensitive communications and electrical distribution systems worldwide. The weather on Earth’s surface is connected with conditions in the lower layers of the atmosphere. Space weather is something new, and has to do with the changing conditions in interplanetary space, the solar wind and the Earth’s magnetosphere. This chapter will focus on Space Weather and we shall begin with the solar wind.