2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/56145I review the issue of string and compactification scales in the weak-coupling regimes of string theory. I explain how in the Brane World scenario a (effectively) two-dimensional transverse space that is hierarchically larger than the string length may replace the conventional `energy desert' described by renormalizable supersymmetric QFT. I comment on the puzzle of unification in this context.13 pages, LateX. Based on talks given at the conferences `22nd Johns Hopkins workshop' (Goteborg, August 1998), `Fundamental interactions: from symmetries to black holes' in honor of Francois Englert (Brussels, March 1999) and `From Planck Scale to Electroweak Scale' (Bad Honnef, April 1999)High Energy Physics - TheoryHigh Energy Physics - Phenomenologytext