CP Nonconservation in Top Quark Production by (Un) Polarized e^+e^- and γ γ Collisions

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 $\gamma\gamma\to{\bar t}t$.