Multiple length scales and screening lengths in QCD at high temperatures

Although asymptotic freedom and perturbative QCD can be used to describe processes which probe the short distance behaviour of the quark-gluon plasma (QGP), perturbation theory breaks down when large distance scales are probed. At large distances the QGP behaves as a confining system and there is some evidence that only color singlet excitations are present at these scales. It is the goal of this article to discuss the basis for these claims. (orig.)