  Observational advances over the last decade have left little doubt that
neutron stars received a large kick velocity (of order a few hundred to a
thousand km/s) at birth. The physical origin of the kicks and the related
supernova asymmetry is one of the central unsolved mysteries of supernova
research. We review the physics of different kick mechanisms, including
hydrodynamically driven, neutrino -- magnetic field driven, and
electromagnetically driven kicks. The viabilities of the different kick
mechanisms are directly related to the other key parameters characterizing
nascent neutron stars, such as the initial magnetic field and the initial spin.
Recent observational constraints on kick mechanisms are also discussed.
