Can we find metal-insulator transitions in 2-dimensional systems?

It has been almost a quarter of a century since the belief of the non–existence metallic state or the non-occurrence of metal-insulator transition for a two–dimensional system in the zero magnetic field was accepted. Nevertheless, during the last decade atypical behavior in semiconductor samples suggested the possibility of those transitions in two dimensions. In this essay we will review some of the experiments that can evidence such transtions and the theoretical models proposed to explain the observed phenomena.