The book deals with the most important areas of the Earth's plasma environment - the polar cap boundaries, the discussion focusing on the key role played by the polar cap and its boundaries in magnetospheric-ionospheric science on both the day and night sides. Identification criteria have been compiled for the magnetospheric boundary layers that are of central importance to current research and to applications in `space weather' activities. The book includes a number of linked themes in solar-terrestrial physics, such as the bow shock, the magnetosheath and the magnetopause; and how the interplanetary magnetic field orientation and the solar wind control the structure and the dynamics of dayside cusp and cleft regions.