Porous silicon (PS), which is derived from silicon, is a viable material option for microelectro mechanical systems (MEMS) and sensors due to its unique properties. The pore size, from nano- to micrometers, can be controlled by varying the formation conditions and decide the mechanical properties and the increased functionality of PS. In this chapter, we discuss the applications of PS in MEMS and sensors. We begin with a discussion on PS formation by electrochemical etching of silicon and the control over the porosity and pore size. Applications of PS for various biosensors, in which the large surface-to-volume ratio of PS helps in improving the sensor performance, are discussed. Silicon/PS composite cantilevers show higher sensitivity as biosensors due both to changes in mechanical properties of the cantilever beam as well as enhanced reaction rate due to larger surface area. The low Young's modulus of PS has also been utilized to improve the sensitivity of MEMS pressure sensors with silicon/PS composite membranes. Experimental results as well as models to understand them have been discussed touching upon the instability of PS and reliability issues with techniques to reduce them. © 2015 Scrivener Publishing LLC.