Three phenomenological analyses on the current unification theories of elementary particle interactions are presented. In Chapter I, the neutral current phenomenology of a class of supersymmetric SU(2) x U(1) x U tilde(1) models is analyzed. A model with the simplest fermion and Higgs structure allowing a realistic mass spectrum is considered first. Its neutral current sector is parametrized in terms of two mixing angles and the strength of the new U tilde(1) interactions. Expressions for low-energy model-independent parameters are derived and compared with those of the standard model. Bounds on the neutral gauge boson masses are obtained from the data for various neutrino interactions, eD scattering, and the asymmetry in ee− .-->. .mu.mu.−. In Chapter II, the evolution of fermion mass in grand unified theories is reexamined. In particular, the question of gauge invariance of mass ratios in left-right asymmetric theories is considered. A simple expression is derived for the evolution of the Higgs-fermion-fermion coupling which essentially governs the scale dependence of fermion mass. At the one loop level the expression is gauge invariant and involves only the representation content of left- and right-handed fermions but not that of Higgs. The corresponding expression for supersymmetric theories is also given. In Chapter III, the production and the subsequent decays of a heavy lepton pair L/sup + -/ near the Z peak in e+e− annihilation are considered as a test of the standard model. The longitudinal polarization is derived from the spin-dependent production cross-section, and the decays L .-->. .pi. nu and L .-->. l nu nu are used as helicity analyzers.