  We consider variations of coupling strengths and mass ratios in and beyond
the Standard Model, in the light of various mechanisms of mass generation. In
four-dimensional unified models, heavy quark and superparticle thresholds and
the electron mass can completely alter the (testable) relation between Delta ln
alpha and Delta ln mu, where mu \equiv m_p/m_e. In extra-dimensional models
where a compactification scale below the fundamental scale is varying, definite
predictions may result even without unification; we examine some models with
Scherk-Schwarz supersymmetry-breaking.

High Energy Physics - Phenomenology;Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory