Constraining the string scale: from Planck to weak and back again

String and field theory ideas have greatly influenced each other since the so-called second string revolution. We review this interrelation paying particular attention to its phenomenological implications. Our guiding principle is the radical shift in the way we think about the fundamental scale, in particular the way in which string models have been able to accommodate values from the Planck MPl ~ 1018 GeV down to the electroweak scale MEW~ TeV.