are faced with 10-dimensional Kaluza-Klein supergravity and the vacuum selection problem.

    -   There are at least five superstring theories: Types IIA, Type IIB, Heterotic $SO(32)$, Heterotic $E(8)\times E(8)$ and Type I $SO(32)$. Which one should be considered?

    -   The theory seems to contain other extended objects besides strings: *D-branes* [@Polchinski:1995mt], NSNS-branes... Why should strings be fundamental [@Townsend:1995gp]?

    The answer to the last two questions lies on the *dualities* that related the different superstring theories and the different extended objects that occur in them [@Schwarz:1996bh]. Dualities are transformations that relate different theories: their spectra, interactions and coupling constants. Their existence allows the mapping of all scattering amplitudes of one theory into those of the other theory and vice-versa. In some cases, the mapping relates the coupling constant of one theory with the inverse coupling constant of the other theory and we talk about the non-perturbative S dualities. In other cases the non-trivial mapping affects only geometrical data of the compactification (moduli) and we talk about perturbative T dualities. These are characteristic of String Theories.

    Dualities are, certainly, not symmetries of a single theory. Instead, they can be seen as symmetries in the space of theories. If two dual theories arise from 