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We calculate the tree-level electroweak precision constraints on a wide classof little Higgs models including: variations of the Littlest Higgs SU(5)/SO(5),SU(6)/Sp(6), and SU(4)^4/SU(3)^4. By performing a global fit to the precisiondata we find that for generic regions of the parameter space the bound on thesymmetry breaking scale f is several TeV, where we have kept the normalizationof f constant in the different models. For example, the ``minimal''implementation of SU(6)/Sp(6) is bounded by f>3.0 TeV throughout most of theparameter space, and SU(4)^4/SU(3)^4 is bounded by f^2 = f_1^2+f_2^2 > (4.2TeV)^2. In certain models, such as SU(4)^4/SU(3)^4, a large f does not directlyimply a large amount of fine tuning since the heavy fermion masses thatcontribute to the Higgs mass can be lowered below f for a carefully chosen setof parameters. We also find that for certain models (or variations) there existregions of parameter space in which the bound on f can be lowered into therange 1-2 TeV. These regions are typically characterized by a small mixingbetween heavy and standard model gauge bosons, and a small (or vanishing)coupling between heavy U(1) gauge bosons and the light fermions. Whether such aregion of parameter space is natural or not is ultimately contingent on the UVcompletion.Comment: 32 pages, 13 figures; revised discussion of SU(4)^4/SU(3)^4 model,  bound on f is slightly highe

Variations of little Higgs models and their electroweak constraints