We consider two separate extensions of the standard model in the hope of understanding the origin of electroweak symmetry breaking and eliminating some of the arbitrariness associated with the Higgs sector of the standard model. Though these approaches are fundamentally different, the underlying theme is that of the role of the top quark in the origin of electroweak symmetry breaking. In the first approach we examine a renormalisable model of top quark condensation. We extract predictions for the top quark's mass using truncated Dyson-Schwinger methods, and find that a satisfactory mass is achieved only for large dynamical scales. We consider an extension to a four generation model in order to reduce the dynamical scale. The model possesses a Peccei-Quinn symmetry which results in an axion. We calculate the axion's mass, which receives contributions from instanton effects associated with the strongly coupling groups introduced by the model, and find that it is acceptably large. In the second approach we analyse the next-to-minimal supersymmetric standard model, in which the radiative breakdown of electroweak symmetry is induced by the top quark's Yukawa coupling. We calculate the upper bound on the mass of the lightest CP-even Higgs scalar in the model. We perform a low energy, phenomenological analysis of the Higgs sector, emphasising signatures which distinguish this model from the minimal supersymmetric standard model. Finally, we perform a supergravity inspired analysis of the model in which soft supersymmetry breaking masses are unified at the grand unification scale and find that the parameter space available at this scale is greatly reduced by the requirement of a sufficiently heavy top quark.