This chapter deals with porous silicon micro-cavities (PSM). Porous silicon was discovered in 1956 by Uhlir, while performing electropolishing experiments on silicon wafers using a hydrofluoric acid that contained electrolyte. This chapter discusses the physics of Porous Silicon (PS), and explains the ways to control its physical parameters to exploit its various properties. This chapter presents the procedure for the production of PS. This chapter presents the structure and optical properties of PS, and gives a general introduction to dielectric multilayers. The mastering of the etching process to form PS layers of designed parameters has allowed the formation of high-quality dielectric multilayers with a competitive technique. A comparison of PS with other Si-based light-emitting systems is performed. This chapter further introduces semiconductor microcavities. Furthermore, this chapter turns to the discussion of PS multilayers and PS microcavities. Some applications of PS microcavities area also highlighted. The recent improvements in the engineering of the PS surface, the demonstration of a full integration of PS-LEDs with both bipolar junction transistor and complementary metal-oxide semiconductor circuits, and the use of microcavities to control the PS spontaneous emission are all recent results that point to brighter future for PS