  After a brief introduction into the general importance of polarization
observables for the analysis of a reaction, the basic density matrix formalism
for the description of polarization phenomena is outlined and illustrated by
explicit examples for spin one-half and one. Then various electromagnetic
processes like photoreactions and electron scattering are discussed in some
detail. Finally, the question of how to determine a complete set of
polarization observables providing the maximum information on a reaction is
considered.
