The behavior of spontaneous emission of atoms in microcavities is significantly different from that in free space. The spontaneous emission is controlled by microcavity, it can be enhanced or restrained, even can become a reversible process. Interaction of the cavity field with an atom is expound by the cavity quantum electrodynamics. We briefly introduce the basic ideas of this subject and its recent progress, also introduce an important application of microcavity-the threshold-less laser.