  We report on an investigation of CP violation in (un)polarized
$e^+e^-\to{\bar t}t$ resulting from an extended neutral Higgs sector or from
the minimal SUSY extension of the Standard Model (SM). We consider c.m.
energies from the ${\bar t}t$ threshold to the TeV range. In addition
sensitivity estimates for CP-violating form factors of the top quark are made.
Further we discuss the prospects of probing Higgs sector CP violationin
$\gamma\gamma\to{\bar t}t$.
