We have seen that the field of string phenomenology has been very
animated
during the past fifteen years and that a lot of progress has been achieved.
There is a very natural justification for this kind of research, namely
to
look for a connection between string theory and physics.
Nevertheless we have seen that this field has also contributed enormously
to the progress towards formulating string theory itself.
It has been claimed that string theory is currently in a similar
situation as particle physics was in the early sixties, in the sense that
there are a lot of experiments without a theory [54].
The only difference is that in string theory, the `experiments'
are just the data we have from string model building.
These data have been fundamental in the discovery of -duality and mirror
symmetry in string theory, furthermore, the conjectured
-duality [10], which relates strong to weak coupling in string theory,
was also originated by asking phenomenological questions.
A symmetry of this sort was also fundamental to exactly *solve*
field theories, even at the non-perturbative level, in the
impressive work of Seiberg and Witten that lead to the
understanding of confinement in a non-trivial 4D field theory[119].
Even though that result
was independent of string theory, the full mathematical treatment was
inherited from recent progress in string compactifications, in particular
the study of mirror symmetries and -duality in Calabi-Yau
compactifications.

Addressing the problems mentioned in the introduction, we have seen that there are five consistent superstring theories and each has thousands or millions of different vacua. It is now believed that the five string theories are related by strong-weak coupling dualities and furthermore, they appear to be different limits of a single underlying fundamental theory, probably in 11D, theory (probably related with membranes or higher dimensional objects such as five-branes), which is yet to be constructed. If this is true it may solve the arbitrariness in the number of fundamental string theories by deriving them from a single theory, on the other hand the number of different vacua seems to be even larger than previously thought. Furthermore, we have also seen that many and probably all Calabi-Yau compactifications are connected. Then it seems that not only the five different theories are unified, but also all the vacua of these theories are unified: since, if they are all connected, we can foresee a mechanism that lifts the degeneracy and select one point in the web of compactifications, something it could not have been done before because they were thought to be disconnected vacua and there was no way to select one. Also the Seiberg-Witten results were reproduced from a field theory approximation of a particular string vacuum, assuming duality to hold. Since the Seiberg-Witten results are robust, this is a non-trivial test for the existence of duality in string theories (for a recent review see [54] ). We are definitely living exciting times in string theory, where not only strings but also higher-dimensional objects are emerging as different pieces of an underlying fundamental theory. It is interesting to remark that the extended objects were originally introduced to represent elementary particles and now, in the brane-world scenario, they can even represent different universes. On the other hand the current optimism for this theory should be taken with the right perspective, none of these achievements can substitute a solid experimental test of the theory, something which may still be very far in the future. However we might be lucky and some discoveries at current and future accelerators, as well as some possible astrophysical results, could provide important clues on the validity of string theory. In particular the idea that the string scale may be as low as the TeV scale may have dramatic experimental consecuences. Even if that scale is larger, it does not have to be as large as the Planck scale and we can say that, at present, the hopes of eventually testing string theory are not unreasonable.

In any case the idea that our Universe is a brane inside a higher dimensional space has changed our view of the physical world. We have seen how the standard scenario where matter lives on the brane and gravity in the bulk is naturally realised in terms of D-branes or also in the Horava-Witten realization of the heterotic string in M-theory. Recently there has been variations in this theme including new solutions of the hierarchy problem and the fact that also gravity can be localized on a brane allowing infinitely large extra dimensions [120] with finite volume and even with infinite volume [121]. The brane-world scenario is allowing several communities of physicists (formal string theorists, string phenomenologists, model builders, hard core phenomenologists, collider experimentalists, table-top experimentalists, cosmologists and astrophysicists) to communicate with each other more than ever in the past. This is a positive signature of a healthy science. We hope all this combined effort will be fruitful in the not to far future.