  Recently there has been renewed interest in the possibility that the Higgs
particle of the Standard Model is a pseudo-Nambu-Goldstone boson. This
development was spurred by the observation that if certain global symmetries
are broken only by the interplay between two or more coupling constants, then
the Higgs mass-squared is free from quadratic divergences at one loop. This
"collective symmetry" breaking is the essential ingredient in little Higgs
theories, which are weakly coupled extensions of the Standard Model with little
or no fine tuning, describing physics up to an energy scale ~10 TeV. Here we
give a pedagogical introduction to little Higgs theories. We review their
structure and phenomenology, focusing mainly on the SU(3) theory, the Minimal
Moose, and the Littlest Higgs as concrete examples.
