  We discuss associated production of a Higgs boson with a pair of $t {\bar t}$
quarks at a future high energy $e^+e^-$ collider. The process $e^+e^-\to t
{\bar t}h$ is particularly sensitive to the presence of new physics and we
consider the MSSM and models with extra dimensions at the $TeV$ scale as
examples.

High Energy Physics - Phenomenology