  The Principle of Naturalness of small parameters of a theory is reviewed.
While quantum field theories constructed from gauge fields and fermions only
are natural, those containing elementary scalar fields are not. In particular
the Higgs boson mass in the Standard Model of electro-weak forces is not stable
against radiative corrections. Two old canonical solutions of this problem are:
(i) where the Higgs boson is a fermion-antifermion composite (technicolour
solution) or otherwise (ii) we need supersymmetry to protect the mass of
elementary Higgs boson from possible large radiative corrections. In recent
years some other mechanisms for electroweak symmetry breaking have been under
intense investigation. These include the little Higgs models and the
gauge-Higgs unification models where the Higgs boson is the zero mode of the
extra-dimensional component of a higher dimensional gauge field. Naturalness
issues of such models are also briefly reviewed.
