  Aim of this paper is to present a brief and basic introduction to polarized
beams and the behaviour of a particle's spin in an electromagnetic field. It is
not intended to give a complete presentation of the spin dynamic theory.
Instead the most importa nt features are given without proof and details not
absolutely necessary are left out. Spin depolarizing mechanisms are discussed
and the cures against depolarizing especially during acceleration as utilized
at COSY are outlined. A list of references is given at the end to help the
reader to find more deep details on spin dynamic theory [1].
