Top Quark Pair Production at Threshold -- Uncertainties and Relativistic Corrections

It is shown how nonrelativistic QCD (NRQCD) can be used to determine next-to-next-to-leading order relativistic and short-distance contributions to the total $t\bar t$ production cross section in the threshold regime at lepton colliders. A recipe for the calculation of all such contributions for the total photon mediated production cross section is presented and a review of the already known Abelian next-to-next-to-leading results is given.