Generated by magnetohydrodynamic processes taking place in its electrically conducting fluid core, which, through magnetic induction, convert the mechanical energy of core convective motions into the ohmic dissipation of the magnetic field, the Earth has had an intrinsic magnetic field and has undergone the temporal and spatial variation including polarity reversals. Since the structure and strength of the Earth's magnetic field observed at the Earth’s surface reflect the chemical, physical and dynamical properties of the Earth’s interior, the existence and variation of the geomagnetic field contain the clues to understanding the Earth’s evolution and its inaccessible deep interior. It is also becoming increasingly recognized that scientists from different disciplines, such as geophysicists, geochemistry, geology and applied mathematicians, must work together in order to solve the unsolved problems in the origin and change of the Earth's magnetic field.