The spontaneous emission of a material can be controlled by placing it in a micron-sized optical cavity. In this paper we introduce the subject and we discuss the realization, the physics and perspective applications of all porous silicon microcavities. The emission properties of the cavities have been characterized as a function of the temperature, of the excitation power and of the response time. Coupled microcavities are demonstrated. Modeling of the structure have been performed on the basis of a transfer matrix approximation.