  Using effective field theory approach, we study the constraints from
electroweak data on the SU(5)/SO(5), SU(6)/SP(6), SU(3)xU(1) little Higgs
models and their variations. We construct an effective theory valid above the
electroweak symmetry breaking scale in these models that includes dimension-six
operators induced by integrating out heavy fields. We calculate the constraints
on the models using the results of hep-ph/0412166, where bounds on arbitrary
linear combinations of flavor and CP conserving dimension-six operators are
given. We present the constraints in terms of the bounds on the masses of heavy
fermions and heavy gauge bosons. The constraints are often stringent, but in
some regions of the parameter space the constraints are mild enough and do not
imply significant fine-tuning.
