  We review the physics of polarization observables in high energy reactions in
general and discuss the status of \NLO one--loop corrections to these
observables in specific. Many high order radiative corrections exists for rates
but not many \NLO radiative corrections exist for polarization observables. The
radiative correction calculations for polarization observables are somewhat
more complicated than those for rates. They tend to be smaller than those for
the rates. In most of the examples we discuss we include mass effects which
significantly complicate the radiative correction calculations. We elaborate a
general scheme which allows one to enumerate the number of independent density
matrix elements in a reaction and provide explicit examples of angular decay
distributions in self--analyzing decays that allow one to experimentally
measure the density matrix elements. We provide examples of reactions where
certain density matrix elements are only populated at \NLO or by mass effects.
In our discussion we concentrate on semileptonic bottom and top quark decays
which are linked to leptonic $ \mu $ and $ \tau $ decays through a Fierz
transformation.
