  We study the dynamics of a quantum particle coupled to dissipative (ohmic)
environments, such as an electron liquid. For some choices of couplings, the
properties of the particle can be described in terms of an effective mass. A
particular case is the three dimensional dirty electron liquid. In other
environments, like the one described by the Caldeira-Leggett model, the
effective mass diverges at low temperatures, and quantum effects are strongly
suppressed. For interactions within this class, arbitrarily weak potentials
lead to localized solutions. Particles bound to external potentials, or moving
in closed orbits, can show a first order transition, between strongly and
weakly localized regimes.
