  In this review article we report on the newest developments in precision
calculations in supersymmetric theories. An important issue related to this
topic is the construction of a regularization scheme preserving simultaneously
gauge invariance and supersymmetry. In this context, we discuss in detail
dimensional reduction in component field formalism as it is currently the
preferred framework employed in the literature. Furthermore, we set special
emphasis on the application of multi-loop calculations to the analysis of gauge
coupling unification, the prediction of thelightest Higgs boson mass and the
computation of the hadronic Higgs production and decay rates in supersymmetric
models. Such precise theoretical calculations up to the fourth order in
perturbation theory are required in order to cope with the expected
experimental accuracy on the one hand, and to enable us to distinguish between
the predictions of the Standard Model and those of supersymmetric theories on
the other hand.
