  We have computed the cross section for $t\bar t$ production by gluon-gluon
fusion at next-to-leading order (NLO) in the QCD coupling, keeping the full
dependence on the $t\bar t$ spins. Furthermore we have determined to the same
order the spin dependent cross sections for the processes $g + q ({\bar q})\to
t {\bar t} + q ({\bar q})$. Together with our previous results for $q + {\bar
q} \to t {\bar t} (g)$ these results allow for predictions, at NLO QCD, of the
hadronic production of $t\bar t$ pairs in a general spin configuration. As an
application we have determined the degree of correlation of the $t$ and $\bar
t$ spins at NLO, using various spin quantisation axes.
