The utilization of mesoporous inorganic materials to improve the dissolution of poorly soluble active pharmaceutical ingredients in oral drug delivery has dramatically increased during the past 10 years. The focus of the present chapter is porous silicon (elemental Si) but the consideration is partly valid also for silicon oxide-based mesoporous materials. Basically, the principle to improve the dissolution behaviour of the payload drugs is the same for both materials: by confining the drug in a space only few times larger than the drug molecules themselves, the physicochemical properties of the drug can be dramatically changed. The confinement leads to stabilized disordered structure (amorphous or nanocrystalline) of the drug with improved dissolution properties. The characteristics of the drug carrier system can be tuned with the physical and chemical properties of the carrier which makes these materials versatile drug carriers. Furthermore, the material can be designed to be suitable for practically any type of payload molecules without compromising the high payload capacity and the stability of the molecules. This edition first published 2013 © 2013 John Wiley & Sons, Ltd.