The dissipative two-level system represents also a paradigm in the study of quantum phase transitions. For a critical value of the coupling to the bath it shows a phase transition from a regime in which the particle is delocalized among the two positions to another in which it is localized in only one of them. The transition is of Kosterlitz–Thouless kind, as can be seen by deriving the renormalization group flow equations for the hopping term.