This thesis examines the effect of inhomogeneities on the magnitude-redshift relationship, focusing particularly on the redshift. We show that a perturbed FRW universe which has the same global behaviour as an exact FRW universe does not have the same light propagation behaviour due to the relationship between redshift and the radial coordinate being influenced by the inhomogeneities introduced. The change in redshift along a light beam depends on the rate of expansion of space so an understanding of the effect of inhomogeneities on the local rate of expansion of space is necessary in order to study the effect of inhomogeneities on redshift. We create a new description of the evolution of matter fluctuations in the weakly non-linear regime which also describes a relationship between the matter density and the local expansion rate of space. This is subsequently used in the development of a new method of calculating the relationship between the angular diameter distance and the redshift of a distant object. We use the new magnitude-redshift relationship in conjunction with the supernova data to estimate cosmological parameters and find that

• to a high level of confidence Ω∆ is non-zero
• most likely flat universe is one with cosmological parameters close to {Ωm) Ω∆} = {0.3, 0.7}.