  We present new theoretical predictions for the t bar t production cross
section at NNLO at the Tevatron and the LHC. We discuss the scale uncertainty
and the errors due to the parton distribution functions (PDFs). For the LHC, we
present a fit formula for the pair production cross section as a function of
the center of mass energy and we provide predictions for the pair production
cross section of a hypothetical heavy fourth generation quark t'.
