  The recent experimental observation of a metal-insulator transition in two
dimensions prompts a re-examination of the theory of disordered interacting
systems. We argue that the existing theory permits the existence of a metallic
phase and propose a number of experiments such as magnetoconductance and
tunnelling in the presence of a parallel field, which should provide diagnostic
tests as to whether a given experimental system is in fact in this regime. We
also comment on a generic flow diagram which predicts a maximum metallic
resistivity.
