We examine here the possibilities for neutrino masses and mixings in extensions of the standard model which are related to the 'anti-grand unification theory.' We shall also examine the effects of coloured scalar fields (which are predicted by the AGUT), on the low-energy phenomenology of the theory. In the standard model the neutrinos are massless, and consequently there are no physically meaningful mixing effects in the lepton sector. However, this is no longer consistent with recent experimental results, which provide strong evidence for non-zero neutrino masses. Hence, when looking for extensions of the standard model it is certainly desirable that they should incorporate neutrino masses. We do not, however, simply wish to put the neutrino masses into the theory by hand, as was done in the standard model for the charged fermion masses. Instead we would like our extension of the SM to provide some explanation for the charged fermion hierarchy, as well as the large neutrino mixing effects which have been observed at solar and atmospheric neutrino experiments. The charged fermion hierarchy is strongly suggestive of the existence of an approximate chiral symmetry, which can lead to the wide range of charged fermion masses which have been observed. Such a symmetry can also be used to constrain neutrino masses and mixings. In this thesis we shall examine one such possibility for a chiral gauge theory, which has already been successfully used to explain the observed charged fermion spectrum. The anti-grand unification theory is an extension of the standard model, with gauge group SMG3 ⊗U(1)f. (0.1) this model has a chiral symmetry which protects the fermions from having fundamental masses and mixings. After this chiral gauge symmetry is broken by Planck scale Higgs fields, it can yield a realistic spectrum of charged fermion masses and mixings, as well as non-zero neutrino masses. However, the neutrino masses are naturally lighter than we expect from the experimental results. Hence, we shall investigate in this thesis the possibility of extending the Higgs scalar sector of the anti-grand unification theory in order to obtain a realistic neutrino spectrum. Whilst the fermion masses are protected by both non-Abelian and Abelian chiral symmetries, we shall see that much of the structure of the theory comes from the Abelian part of the gauge group. Hence this suggests that it is possible to construct Abelian extensions of the standard model which can explain the fermion masses and mixings. We shall examine this possibility in this thesis, and find that such theories can indeed be constructed. The new scalars which we introduce to incorporate realistic neutrino masses and mixings affect the low-energy phenomenology of the theory only through the fermion masses and mixings. However, there also exist coloured scalars which are partners of the SM WS Higgs field in the AGUT, which in previous analyses of the model have always been taken to be too heavy to affect the low-energy phenomenology of the theory. This, however, is not necessarily the case. We examine here whether these coloured scalars can affect the self-consistency of the theory, and whether (if they are light) they conflict with experimental constraints. In particular we shall see that they lead to tree-level flavour changing currents. The contribution to FCNC is however sufficiently suppressed that they can be reasonably light (about 0.5 TeV) without causing any conflict with experiment.