Supersymmetric Grand Unified Theories are an attractive extension of the Standard Model of Particle Physics. They unify three of the four fundamental forces which leads to an unification of the matter fields into representations of the unified gauge group. Therefore the Yukawa couplings of, for example, the down-type quarks and charged leptons are not independent anymore. In the simplest setup these relations turn out to be challenged by experiment. In this thesis we will revise the standard Yukawa relations in Grand Unified Theories. This analysis either points to a certain class of supersymmetry breaking or it can be understood as a hint towards newly derived Yukawa relations in unified theories. We also discuss various Yukawa matrix textures predicting the right unitarity triangle angle to be within its experimental uncertainty. Finally we construct two flavour models predicting all CP phases in the quark and lepton sector. These models include some of the newly proposed Yukawa relations giving good agreement with the observed fermion masses.