  Observations over the last decade have shown that neutron stars receive a
large kick velocity (of order a 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.
