  The main purpose of this thesis is to discuss the impact of electroweak
precision measurements on the present knowledge of particle physics, both in
the framework of the Standard Model and of its most straightforward extensions.
In Chapter 1 we give a general overview of the electroweak sector of the SM and
of SUSY phenomenology. In Chapter 2 we deal with different definitions of the
electroweak mixing angle, with particular attention to the relation between the
phenomenological angle $\theta$ and the MS parameter $\hat\theta$. In Chapter 3
we use results found in the previous chapter to discuss in detail how both the
decoupling and the non-decoupling approach to the running of coupling constants
in the MS scheme produce the same numerical value of $m_{GUT}$, despite of the
different initial conditions. In Chapter 4 we derive simple analytical formulas
for the contribution of a chargino almost degenerate with the lightest
neutralino to electroweak observables, showing that in the case of wino
domination the study of oblique corrections allows a concrete improvement of
experimental bounds on chargino mass. Finally, in Chapter 5 we consider the
possibility of extra fermion generations, proving that they are strongly
disfavoured by the present experimental data if all particles are heavier that
Z-boson, while for the specific case of the extra neutrino around 50 GeV in
mass (still allowed by experimental data) the quality of the fit is not worse
than the SM.
