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The reduction of couplings idea consists in the search for renormalizationgroup invariant relations between seemingly independent parameters of a theorythat hold to all orders of perturbation theory. This concept can be applied to$N=1$ GUTs and make them all-loop finite. In the first part of this thesis,after a review of the reduction of couplings method and the properties offiniteness, four models are analysed: a reduced version of the minimal $N=1$$SU(5)$, a finite $N=1$ $SU(5)$, a two-loop finite $N=1$ $SU(3)^3$ and areduced version of the MSSM. An update in the phenomenological evaluation isthe improved light Higgs-boson mass prediction as provided by the FeynHiggscode. The first three models predict heavy SUSY spectra (produced using theFeynHiggs and SPheno codes) that start just above the TeV scale, consistentwith the non-observation LHC results, while the reduced MSSM results in apseudoscalar Higgs boson mass $M_A$ that is ruled out by recent results of theATLAS experiment. The three models have very dim prospects of discovery at theHL-LHC. The FCC-hh, however, could be able to test large parts of theirpredicted spectra.  In the second part of the thesis, after a review of the Coset SpaceDimensional Reduction, an extension of the SM is presented that results fromthe dimensional reduction of the $N=1$, $10D$ $E_8$ group over a $M_4 \timesB_0/ \mathbf{Z}_3 $ space, where $B_0$ is the nearly-K\"ahler manifold$SU(3)/U(1)\times U(1)$ and $\mathbf{Z}_3$ is a discrete group on $B_0$. Usingthe Wilson breaking mechanism we get a $4D$ $N=1$ $SU(3)^3$ model plus twoglobal $U(1)$s. Below the GUT scale we have a two Higgs doublet model in asplit-like SUSY version of the SM which is phenomenologically consistent, sinceit produces masses of the top, bottom and light Higgs particles within theexperimental limits and predicts the LSP $\sim 1500 GeV$.

Phenomenological Consequences of Supersymmetric Theories from Dimensional Reduction and Reduction of Couplings