  The dynamics of {\it light} fermions propagating in a spatial direction at
high temperatures can be described effectively by a two--dimensional
Schr\"odinger equation with {\it heavy} effective mass $m_{\rm eff} = \pi T$.
Starting from QED, we discuss the transition from three-- to two--dimensional
positronium discussing the latter in detail including relativistic effects. In
the case of QCD the problem is similar to that of heavy quarkonium. Our
effective potential contains the usual Coulomb and confining parts as well as a
perturbative spin--spin interaction. The resulting $\bar q q$ ``wave functions"
reproduce recent lattice data for the $\rho$ and $\pi$ channels. The physical
meaning of such `confinement' is related to the non--trivial magnetic
interaction of color currents in the quark--gluon plasma. Our results do not
contradict the idea that the normal electric interaction of color charges is
screened and produces no bound states in the usual sense.
