The key mission objective of the Solar Orbiter is to study the Sun from close-up (45 solar radii, or 0.21 AU) in an orbit tuned to solar rotation in order to examine the solar surface and the space above from a co-rotating vantage point at high spatial resolution. Solar Orbiter will also provide images of the Sun's polar regions from heliographic latitudes as high as 38 degrees. The strawman payload encompasses two instrument packages: Solar remote-sensing instruments: EUV full-sun and high resolution imager, high-resolution EUV spectrometer, high-resolution and full-sun visible light telescope and magnetograph, EUV and visible-light coronagraphs, radiometers. Heliospheric instruments: solar wind analyzer, radio and plasma wave analyzer, magnetometer, energetic particle detectors, interplanetary dust detector, neutral particle detector, solar neutron detector. To reach its novel orbit, Solar Orbiter will make use of low-thrust solar electric propulsion (SEP) interleaved by Earth and Venus gravity assists. Solar Orbiter was selected by ESA's Science Programme Committee (SPC) in October 2000 as a Flexi-mission, to be implemented after the BepiColombo cornerstone mission to Mercury before 2013. This paper summarizes the science to be addressed with the Solar Orbiter, followed by brief descriptions of the strawman payload, the mission profile, and the spacecraft and ground segment designs.