After a brief historical introduction, this chapter describes the regions of the heliosphere. Fundamental parameters of the solar wind are presented and their variations during the activity cycle are discussed. The solar wind consists of slow and fast streams originating in the magnetically closed (as magnetic field arcs) as well as open corona (in corona holes). The sources of the solar wind are explained. Magnetic funnels in the network are important. The basal acceleration of the solar wind is due to the solar electric field and is enhanced by momentum transfer from Alfvén waves. Coronal mass ejections, turbulence and wave phenomena in the solar wind are discussed. Emphasis is placed on microphysics and kinetic processes as well as velocity distributions of protons and electrons. Basic concepts from plasma physics and plasma instabilities are also explained.