The major part of microfluidic devices nowadays consists of a dense material that defines the fluidic structure. A generic fabrication method enabling the production of completely porous micro devices with user-defined channel networks is developed. The channel walls can be used as a (selective) barrier for transport. In this conceptual article the focus is on the opportunities offered by this approach. Fabrication method and applicable materials are briefly discussed. Two applications of porous micro devices are shown: a micro gas-liquid contactor and a porous channel emulsificator that can be used to make monodispersed droplets. For both devices a short comparison with alternative concepts is drawn. Finally, key areas for the presented technology are addressed.