Abstract: Naturalness of the electroweak symmetry breaking scale requires new physics at around 1 TeV. On the other hand, constraints from precision electroweak measurements reveal no evidence for new physics up to 5 - 10 TeV, which creates a tension with the naturalness requirement, known as the "little hierarchy problem." Little Higgs theories are an attempt to address this problem. In little Higgs theories, the Higgs doublet(s) are pseudo-Nambu-Goldstone bosons with the special property such that the cutoff of the theory can be raised up to 10 TeV, beyond the scales probed by the current electroweak precision data. However, new TeV scale particles that are required to cancel the one-loop quadratic divergence can also give large contributions to the electroweak observables and re-introduce the fine-tuning problem. I will discuss a general class of the little Higgs theories with a new symmetry, called T-parity, which eliminates all the tree-level contributions to the electroweak observables and hence makes these theories natural. The existence of T-parity has drastic impacts on phenomenology of little Higgs theories. The T-odd particles need to be pair-produced and will cascade down to the lightest T-odd particle which is stable. A neutral lightest T-odd particle gives rise to missing energy signals at the colliders that can mimic supersymmetry. It also serves as a good dark matter candidate.