Symmetry Breaking in Extra Dimensions

In this thesis we analyze the problem of symmetry breaking in models with extra dimensions compactified on orbifolds. In the first chapter we briefly review the main symmetry breaking mechanisms peculiar of extra dimensions such as the Scherk-Schwarz mechanism, the Hosotani mechanism and the orbifold projection. In the second chapter we study the most general boundary conditions for fields on the orbifold S^1/Z_2 and we apply them to gauge and SUSY breaking. In the third chapter we focus on flavour symmetry and we present a six dimensional toy model for two generations that can solve the fermion hierarchy problem.