We discuss the spontaneous emission of quantum well excitons in a monolithic microcav- ity. When the quantum well is excited by a nonresonant pump wave at high above the bandgap, the incoherent spontaneous emission is concentrated on the single resonant mode and the laser threshold is reduced by many orders of magnitude. When the quantum well exciton is excited by a resonant pump wave, the coherent spontaneous emission based on a "microcavity exciton-polariton" is observed. The spectral linewidth and the polariza- tion of the pump wave are preserved and the coupling efficiency into the single resonant mode approaches 100%. The microcavity-induced normal mode splitting is observed in the frequency domain by photoexcitation spectrum measurements and in the time domain by pump-probe measurements.