Aspects of symmetry breaking in Grand Unified Theories

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.