Higgs Radiation off Top Quarks in $e^{+}e^{-}$ Collisions

The strength of the Yukawa coupling of the leptons and quarks with the Higgs boson is uniquely determined by the Higgs mechanism for generating masses in the Standard Model. The top-Higgs coupling can be measured directly in the process e^+e^- --> t tbar H. This process therefore provides a fundamental test of the Higgs mechanism. Extending earlier analyses carried out in the Born approximation, we have determined the cross section for this process including QCD corrections, which turn out to be important.