  In this thesis we discuss some aspects concerning the construction of a 4D
effective theory derived from higher dimensional (in particular 6D) models. The
first part is devoted to the study of how the heavy Kaluza-Klein modes
contribute to the low energy dynamics of the light modes. The second part
concerns the analysis of the spectrum arising from non standard
compactifications of 6D minimal gauged supergravities, involving a warp factor
and conical defects in the internal manifold. We also review some of the
background material.
