  We consider variation 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, variations in charged particle thresholds,
light quark masses 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.
