We analyze elementary properties of exciton and polariton lasers - dev
ices that generate coherent optical and matter waves using final-state
stimulation of exciton-phonon scattering. First we discuss the relati
on between the conditions for the onset of equilibrium and nonequilibr
ium excitonic condensates. Provided that the thermal de Broglie wavele
ngth lambda(T) exceeds the exciton Bohr radius a(B), an exciton laser
operates without electronic population inversion. In contrast to previ
ous proposals, this is a different type of laser without inversion whi
ch utilizes many-body coherences. When the excitonic character of the
polariton branch vanishes, a polariton laser becomes indistinguishable
from a photon laser.