## ABSTRACT

This chapter discusses the generic ‘hierarchy problem’ that is inherent in all guts, namely the huge disparity between the (huge) energy scale at which the grand unified theory (gut) symmetry is broken and the (TeV) energy scale characterizing the familiar breaking of the electroweak symmetry. In a supergravity theory emerging from string theory the GUT scale may plausibly be near the string scale, but the origin of the electroweak scale remains problematic. At the time of writing the favoured solution is that the electroweak symmetry breaking is driven by radiative corrections, in which the mass-squared term for the electroweak scalar ‘runs’ from a positive value at the gut scale to a negative value at the TeV scale, thereby inducing the spontaneous breaking of the electroweak symmetry. The chapter formulates the renormalization group equations that determine whether the electroweak symmetry is broken by radiative corrections.