String theory is an extension of quantum field theory which incorporates quantum gravity. Recently, 10-dimensional 
string backgrounds were found to reproduce the MSSM at low energies. However, this is only an intermediate step 
as we would like to answer many questions which are left unanswered by the MSSM. 
Many of these have to do the intrinsic difficulties of the theory of quantum gravity. 
But by effectively turning on gravity in 4-dimensions we can fulfil many
of the phenomenological requirements which have a direct impact on particle physics. Interestingly
enough, string theory allows us such freedom and in the process provides and intuitive geometric
picture through the brane worlds approach.
One of the key problems that one would like to address is the issue of gauge coupling unification.
The most natural realization of such models is still some type of GUT. In particular, one would like to
have realizations of such models in type II-B string theory where most of the recent progress in moduli
stabilization, mediation of supersymmetry breaking and other issues have taken place. Attempts have
been made at constructing D-brane GUT models which suffer from a number of diffculties like the
lack of spinor representation for SO(10) or the perturbative vanishing of the top quark Yukawa
couplings for SU(5) models. In fact, these di?culties arise because the past constructions relied
on local 7-branes. Such obstacles can be avoided by considering mutually non-local 7-branes. This
enlarged class of models goes by the name of F-theory.