  Using non-relativistic effective theories, new next-to-next-to-leading order
(NNLO) QCD corrections to the total $t\bar t$ production cross section at the
Linear Collider have been calculated recently. In this article the NNLO
calculations of several groups are compared and the remaining uncertainties are
discussed. The theoretical prospects for an accurate determination of top quark
mass parameters are discussed in detail. An outlook on possible future
improvements is given.
