The strong coupling of excitons and photons inside a semiconductor microcavity forms a trap in k-space for the coupled polaritons. This trap produces new behaviour from the strong parametric scattering of polaritons on the walls of the trap, from condensation of polaritons at the bottom of the trap and from the bottleneck in relaxation at the neck of the trap. We predict the operation of polariton lasers pumped non-resonantly. either electrically or optically, based on efficient scattering of polaritons into the trap. (C) 2002 Elsevier Science B.V. All rights reserved.