  In this talk we discuss the process e+e- -> t t-bar near threshold. In
particular we discuss a quark mass definition which is a generalization of the
static PS mass. The new definition allows us to calculate recoil corrections to
the static PS mass. Using this result we calculate the cross section of e+e- ->
t t-bar near threshold at NNLO accuracy adopting three alternative approaches,
namely (1) fixing the pole mass, (2) fixing the PS mass, and (3) fixing the new
mass which we call the PS-bar mass. We demonstrate that perturbative
predictions for the cross section become much more stable if we use the PS or
the PS-bar mass for the calculations. A careful analysis suggests that the top
quark mass can be extracted from a threshold scan at NLC with an accuracy of
about 100-200 MeV.
