  The Standard Model is the low-energy limit of a microscopic theory which
includes extra dimensions and new symmetries. A part of my thesis consisted in
constructing a new class of models with two extra dimensions. We showed that
these models can drive a dynamical symmetry breaking in four dimensions. In
particular, they are dual to models with four fermions which need a
non-perturbative treatment. We also studied supersymmetry and its local
version, supergravity, in string theory inspired models with ten dimensions.
Extra fields called moduli are present after dimensional reduction and are
generically flat directions of the total potential. We constructed a model of
stabilisation for the moduli in which the supersymmetry breaking yields a light
gravitino mass of order 10 TeV. The cosmological constant can be adjusted to be
very small. Such models exhibit a promising phenomenology.
