  We reconsider the issue of spontaneous symmetry breaking in SO(10) grand
unified theories. The emphasis is put on the quest for the minimal Higgs sector
leading to a phenomenologically viable breaking to the standard model gauge
group. Longstanding results claimed that nonsupersymmetric SO(10) models with
just the adjoint representation triggering the first stage of the breaking
cannot provide a successful gauge unification. The main result of this thesis
is the observation that this no-go is an artifact of the tree level potential
and that quantum corrections opens in a natural way the vacuum patterns
favoured by gauge coupling unification. An analogous no-go, preventing the
breaking of SO(10) at the renormalizable level with representations up to the
adjoint, holds in the supersymmetric case as well. In this respect we show that
a possible way-out is provided by considering the flipped SO(10) embedding of
the hypercharge. Finally, the case is made for the hunting of the minimal
SO(10) theory.
